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FOCAL PLANE

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Originally appearing in Volume V16, Page 649 of the 1911 Encyclopedia Britannica.
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FOCAL PLANE. (NTRAMCI ROOM. WA? ER TANK. et James Walker r86a J. N. Douglass O.M.Poe 1874 Wolf Rock. Dhu Heartach. FIG. 16.—Spectacle Reef. FIG. 17.—Ar'men. Bishop Rock and Eddystone towers has 1905); Jument d'Ouessant (France, 1907); and Roche Bonne (France, building 1910). Jointing of Stones in Rock Towers.—Various methods of jointing the stones in rock towers are shown in figs. 6 and 22. The great distinction between the towers built by successive engineers to the 'Trinity House and other rock lighthouses is that, in the former the stones of each course are dovetailed together both laterally and vertically and are not connected by metal or wooden pins and wedges and dowled as in most other cases. This dove-tail method was first adopted at the Hanois Rock at the suggestion of Nicholas Douglass. On the upper face, one side and at one end of each block is a dovetailed projection. On the under face and the other side and end, corresponding dovetailedeffect of waves on the been noted above. Land Structures for Lighthouses.—The erection of lighthouse towers and other buildings on land presents no difficulties of construction, and such buildings are of ordinary architectural character. It will therefore be unnecessary to refer to them in detail. Attention is directed to the Phare d'Eckmuhl at Penmarc'h (Finistere), completed in 1897. The cost of this magnificent structure, 207 ft. in height from the ground, was largely defrayed by a bequest of £12,000 left by the marquis de Blocqueville. It is constructed entirely of granite, and is octagonal in plan. The total cost of the tower and other light-house buildings amounted to £16,000. Name of Structure. Total Cost. Cub. ft. Cost per cub. ft. of Masonry. Eddystone, Smeaton (1759) . x,40,000 0 0 13,.343 £2 19 Ili Bell Rock, Firth of Forth (1811) . 55,619 12 I 28,530 119 0 Skerryvore, west coast of Scotland (1844) 72,200 I1 6 58,58o I 4 Bishop Rock, first granite tower (1858) 34,559 18 9 35,209 0 19 71 Smalls, Bristol Channel (1861) . 50,124 r1 8 46,386 I i 74 Hanois, Alderney (1862) . . 25,296 0 0 24,542 I 0 74 Wolf Rock, Land's End (1869) . 62,726 0 0 59,070 1 I 3 Dhu Heartach, west coast of Scotland (1872) 72,584 9 7 42,050 114 6 Longships, Land's End (1872) . 43,869 8 11 47,610 0 18 5 Eddystone, Douglass (1882) . 59,255 0 0 65,198 0 18 2 Bishop Rock, strengthening and part reconstruction (1887) 64,889 0 0 45,080 1 8 9 Great Basses, Ceylon (1873) . . 63,56o 0 0 47,819 I 6 7 Minot's Ledge, Boston, Mass. (186o) . . 62,500 0 0 36,322 117 2 Spectacle Reef, Lake Huron (1874) 78,125 0 0 42,742 I 16 2 Armen, France (1881) 37,692 0 0 32,400 13 3 Fastnet, Ireland (1904) . 79,000 0 0 62,600 I 5 51 recesses are formed with just sufficient clearance for the raised bands to enter in setting (fig. 23). The cement mortar in the joint formed between the faces so locks the dovetails that the stones cannot be separated without breaking (fig. 24). Effect of 1Vaves.—The wave stroke to which rock lighthouse towers are exposed is often considerable. At the Dhu Heartach, during the erection of the tower, 14 joggled stones, each of 2 tons weight, were washed away after having been set in cement at a height of 37 ft. above high water, and similar damage was done during the construction of the Bell Rock tower. The The tower at Ile Vierge (Finistere), completed in 1902, has an elevation of 247 ft. from the ground level to the focal plane, and is probably the highest structure of its kind in the world. The brick tower, constructed at Spurn Point, at the entrance to the Humber and completed in 1895, replaced an earlier structure erected by Smeaton at the end of the 18th century. The existing tower is constructed on a foundation consisting of concrete cylinders sunk in the shingle beach. The focal plane of the light is elevated 12o ft. above high water. Besides being built of stone or brick, land towers are frequently constructed of cast iron plates or open steel-work with a view to economy. Fine examples of the former are to be found in many British colonies and elsewhere, that on Dassen Island (Cape of Good Hope), 105 ft. in height to the focal plane, being typical (fig. 25). Many openwork structures up to 200 ft. in height have been built. Recent examples are the towers erected at Cape San Thorne (Brazil) in 1882, 148 ft. in height (fig. 26), Mocha (Red Sea) in 1903, 18o ft. and Sanganeb Reef (Red Sea) 1906, 165 ft. in height to the focal plane. 3. OPTICAL APPARATUS.—OptiCal apparatus in lighthouses is required for one or other of three distinct purposes: (1) the concentration of the rays derived from the light source into a belt of light distributed evenly around the horizon, condensation in the vertical plane only being employed; (2) the concentration of the rays both vertically and horizontally into a pencil or cone of small angle directed towards the horizon and caused to revolve about the light source as a centre, thus producing a flashing light; and (3) the condensation of the light in the vertical plane and also in the horizontal plane in such a manner as to concen- trate the rays over a limited azimuth only. Apparatus falling under the first category produce a fixed light, and further distinction can be provided in this class by mechanical means of occultation, resulting in the pro- duction of an occulting or intermittent light. Apparatus included in the second class are usually employed to produce flashing lights, but sometimes the dual condensation is taken advantage of to produce a fixed pencil of rays thrown towards the horizon for the purpose of marking an isolated danger or the limits of a narrow channel. Such lights are best described by the French term feux de direction. Catoptric apparatus, by which dual condensation is produced, are moreover sometimes used for fixed lights, the light pencils overlapping each other in azimuth. Apparatus of the third class are employed for sector lights or those throwing a beam of light over a wider azimuth than can be conveniently covered by an apparatus of the second class, and for reinforcing the beam of light emergent from a fixed apparatus in any required direction. The above classification of apparatus depends on the resultant effect of the optical elements. Another classification divides the instruments themselves into three classes: (a) catoptric, (b) dioptric and (c) catadioptric. Catoptric apparatus are those by which the light rays are reflected only from the faces of incidence, such as silvered mirrors of plane, spherical, parabolic or other profile. Dioptric elements are those in which the light rays pass through the optical glass, suffering refraction at the incident and emergent faces (fig. 27). Catadioptric elements are combined of the two foregoing and consist of optical prisms in which the light rays suffer refraction at the incident face, total internal reflexion at a second face and again refraction on emergence at the third face (fig. 28). The object of these several forms of optical apparatus is notonly to produce characteristics or distinctions in lights to enable them to be readily recognized by mariners, but to utilize the light rays in directions above and below the horizontal plane, and also, in the case of revolving or flashing lights, in azimuths not requiring to be illuminated for strengthening the beam in the direction of the mariner. It will be seen that the effective condensation in flashing lights is very much greater than in fixed belts, thus enabling higher intensities to be obtained by the use of flashing lights than with fixed apparatus. Catoptric System.—Parabolic reflectors, consisting of small facets of silvered glass set in plaster of Paris, were first used about the year 1763 in some of the Mersey lights by Mr Hutchinson, then dock master at Liverpool (fig. 29). Spherical metallic reflectors were introduced in France in 1781, followed by parabolic reflectors on silvered copper in 1790 in England and France, and in Scotland in 1803. The earlier lights were of fixed type, a number of reflectors being arranged on a frame or stand in such a manner that the pencils of emergent rays overlapped and thus illuminated the whole horizon continuously. in 1783 the first revolving light was erected at Marstrand inSweden. Similar apparatus were installed at Cordouan (1790), Flamborough Head (18o6) and at the Bell Rock (1811). To produce arevolving or flashing light the reflectors were fixed on a revolving carriage having several faces. Three or more reflectors in a face were set with their axes parallel. A type of parabolic reflector now in use is shown in fig. 30. The sizes in general use vary from 21 in. to 24 in. diameter. These instruments are still largely used for light-vessel illumination, and a few important land lights are at the present time of catoptric type, including those at St Agnes (Scilly Islands), Cromer and St Anthony (Falmouth). Dioptric System.—The first adaptation of dioptric lenses to light-houses is probably due to T. Rogers, who used lenses at one of the Portland lighthouses between 1786 and 1i90. Subsequently lenses by the same maker were used at Howth, Waterford and the North Foreland. Count Buffon had in 1748 proposed to grind out of a solid piece of glass a lens in steps or concentric zones in order to reduce the thickness to a minimum (fig. 31). Condorcet in 1773 and Sir D. Brewster in 1811 designed built-up lenses consisting of stepped annular rings. Neither of these proposals, however, was intended to apply to lighthouse purposes. In 1822 Augustin Fresnel constructed a built-up annular lens in which the centres of curvature of the different rings receded from the axis according to their distances from the centre, so as practically to eliminate spherical aberration; the only spherical surface being the small central part or " bull's eye " (fig. 32). These lenses were intended for revolving lights only. Fresnel next produced his cylindric refractor or lens belt, consisting 2o.—American Shoal Lighthouse, Florida. 7- 22-Om. of a zone of glass generated by the revolution round a vertical axis of a medial section of the annular lens (fig. 33). The lens belt condensed and parallelized the light rays in the vertical plane only, while the annular lens does so in every plane. The first revolving light constructed from Fresnel's designs was erected at the Cordouan lighthouse in 1823. It consisted of 8 panels of annular lenses placed round the lamp at a focal distance of 920 mm. To utilize the light, Dhu Heal wen, 1st Course. Chickens, 6th Course. sCA.C~oF n+E COUA$CS., e which would otherwise escape above the lenses, Fresnel introduced a series of 8 plain silvered mirrors, on which the light was thrown by a system of lenses. At a subsequent period mirrors were also placed in the lower part of the optic. The apparatus was revolved by clock-work. This optic embodied the first combination of dioptric and catoptric elements in one design (fig. 34). In the following year Fresnel designed a dioptric lens with catoptric mirrors for fixed light, which was the first of its kind installed in a lighthouse. It was erected at the Chassiron lighthouse in 1827 (fig. 35). This combination is geometrically perfect, but not so practically on account of the great Stone (Wolf Rock). of Dovetail. loss of light entailed by metallic reflection which is at least 25% greater than the system described under. Before his death in 1827 Fresnel devised his totally reflecting or catadioptric prisms to take the place of the silvered reflectors previously used above and below the lens elements (fig. 28). The ray Fi falling on the prismoidal ring ABC is refracted in the direction i r and meeting the face AB at an angle of incidence greater than the critical, is totally reflected in the direction r e emerging after second refraction in a horizontal direction. Fresnel devised these prisms for use in fixed light apparatus, but the principle was, at a later 0 I date, also applied to flashing lights, in the first instance by T. Stevenson. Both the dioptric lens and catadioptric prism invented by Fresnel are still in general use, the mathematical calculations of the great French designer still forming the basis upon which lighthouse opticians work. Fresnel also designed a form of fixed and flashing light in which the distinction of a fixed light, varied by flashes, was produced by placing panels of straight refracting prisms in a vertical position on a revolving carriage outside the fixed light apparatus. The revolu- tion of the upright prisms periodically in- creased the power of the beam, by condensa- tion of the rays emergent from the fixed apparatus, in the sue' horizontal plane. The lens segments in Fresnel's early appara- FIG. 27.-Dioptric Prism. tus were of polygonal form instead of cylindrical, but subsequently manufacturers succeeded in grinding glass in cylindrical nags of the form now used. The first apparatus of this description was made by Messrs Cookson of Newcastle in 1836 at the suggestion of Alan Stevenson and erected at Inchkeith. In 1825 the French Commission des Phares decided upon the exclusive use of lenticular apparatus in its service. Br The Scottish Lighthouse -E -e spy. Board followed with the Inchkeith revolving apparatus in 1835 and the Isle of May fixed optic in 1836. In the latter instrument Alan Stevenson introduced helical frames for holding the glass prisms in place, thus avoiding complete obstruction of the light rays in any azimuth. The first dioptric light erected by the Trinity House was that formerly at Start Point in Devon-shire, constructed in 1836. Catadioptric or reflecting FIG. 28.-Catadioptric or Reflecting prisms for revolving lights Prism. were not used until 185o, when Alan Stevenson designed them for the North Ronaldshay lighthouse. Bell Rock Flom. Eddystone, 12th Course, Smeaton's Tower. Eddystone, 48th Course, Douglass Tower. .e /1m w sl T _Fogel Pang - F Dooptric Mirror.—The next important improvement in lighthouse I intervals. The cam-wheel is actuated by means of a weight or optical work was the invention of the dioptric spherical mirror by Mr (afterwards Sir) J. T. Chance in 1862. The zones or prisms are generated round a vertical axis and divided into segments. This form of mirror is still in general use (figs. 36 and 37). Azimuthal Condensing Prisms.—Previous to 185o all apparatus were designed to emit light of equal power in every azimuth either constantly or periodic- ally. The only excep- tion was where a light was situated on a • stretch of coast where a mirror could be placed behind the flame to utilize the rays, which would otherwise pass land-ward, and reflect them the ack, flpassmeing and rol ns in a seaward direction. In order to increase the intensity of lights in certain azimuths T. Stevenson devised his azimuthal condensing prisms which, in various forms and methods of application, have been largely used for the purpose of strengthening the light rays in required directions as, for instance, where coloured sectors are provided. Applications of this system will be referred to subsequently. Optical Glass for Lighthouses.—In the early days of lens lights the only glass used for the prisms was made in France at the St Gobain and Premontre works, which have long been celebrated for the high quality of optical glass produced. The early dioptric lights erected in the United Kingdom, some 13 in all, were made by Messrs Cook-son of South Shields, who were instructed by Leonor Fresnel, the brother of Augustin. At first they tried to mould the lens and then to grind it out of one thick sheet of glass. The successors of the Cookson firm abandoned the manufacture of lenses in 1845, and the firm of Letourneau & Lepaute of Paris again became the monopolists. In 185o Messrs Chance Bros. & Co. of Birmingham began the manufacture of optical glass, assisted by M. Tabouret, a French expert who had been a colleague of Augustin Fresnel himself. The first light made by the firm was shown at the Great Exhibition of 1851, since when numerous dioptric apparatus have been constructed by Messrs Chance, who are, at this time, the only manufacturers of lighthouse glass in the United Kingdom. Most of the glass used for apparatus constructed in France is manufactured at St Gobain. Some of the glass used by German constructors is made at Rathenow in Prussia and Goslar in the Harz. The glass generally employed for lighthouse optics has for its refractive index a mean value of µ=1.51, the corresponding critical angle being 41 ° 30'. Messrs Chance have used dense flint glass for the upper and lower refracting rings of high angle lenses and for dioptric mirrors in certain cases. This glass has a value of µ=1.62 with critical angle 38° 5'. Occulting Lights.—During the last 25 years of the 19th century the disadvantages of fixed lights became more and more apparent. At the present day the practice of installing such, except occasionally in the case of the smaller and less important of harbour or river lights, has practically ceased. The necessity for providing a distinctive characteristic for every light when possible has led to the conversion of many of the fixed-light apparatus of earlier years into occulting lights, and often to their supersession by more modern and powerful flashing apparatus. An occulting apparatus in general use consists of a cylindrical screen, fitting over the burner, rapidly lowered and raised by means of a cam-wheel at stated spring clock. Varying characteristics may be procured by means of such a contrivance—single, double, triple or other systems of occultation. The eclipses or periods of darkness bear much the same relation to the times of illumination as do the flashes to the eclipses in a revolving or flashing light. In the case of a first-order fixed light the cost of conversion to an occulting characteristic does not exceed £250 to f300. With apparatus illuminated by gas the occultations may be produced by successively raising and lowering the gas at stated intervals. Another form of occulting mechanism employed consists of a series of vertical screens mounted on a carriage and revolving round the burner. The carriage is rotated on rollers or ball bearings or carried upon a small mercury float. The usual driving mechanism employed is a spring clock. " Otter " screens are used in cases when it is desired to produce different periods of occultations in two or more positions in azimuth in order to differentiate sectors marking shoals, &c. The screens are of sheet metal blacked and arranged vertically, some what in the manner of the laths of a venetian blind, and operated by mechanical means. Leading Lights.—In the case of lights designed to act as a lead through a narrow channel or as direction lights, it is undesirable to employ a flashing apparatus. Fixed-light optics are employed to meet such cases, and are generally fitted with occulting mechanism A typical apparatus of this description is that at Gage Roads, Fremantle, West Australia (fig. 38). The occulting bright light covers the fair-way, and is flanked by sectors of occulting red and green light marking dangers and Plan intensified by vertical con- densing prisms. A good FIG. 34.—Fresnel's Revolving example of a holophotal Apparatus at Cordouan Lighthouse. direction light was exhibited at the 1900 Paris Exhibition, and afterwards erected at Suzac lighthouse (France). The light consists of an annular lens 500 mm. focal distance, of 18o° horizontal angle and 157° vertical, with a mirror of 18o° at the back. The lens throw,s a red beam of about 42° amplitude in azimuth, and 50,000 candle-power over a narrow channel. The illuminant is an incandesce It petroleum vapour burner. Holophotal direction lenses of this type can only be applied where the sector to be marked is of comparatively small • angle. Silvered metallic mirrors of parabolic form are also used for the purpose. The use of • single direction lights frequently renders the construction of separate towers for leading lights unnecessary. If two distinct lights are employed to in. dicate the line of navigation through a channel or between dangers they must be sufficiently far apart to afford a good lead, the front or seaward light being situated at a lower elevation than the rear or landward one. Coloured Lights.—Colour is used as seldom as possible as a distinction, entailing as it does a considerable reduction in the power of the light. It is necessary in some instances for differentiating sectors over dangers ant for harbour lighting purposes. The use of coloured lights as alternating flashes for lighthouse lights is not to be come mended, on account of the unequal absorption of the coloured section at Pendeen in Cornwall is shown in fig. 39; and fig. 55 (Plate I.) illustrates a double flashing first order light at Pachena Point in British Columbia. Hopkinson's system has been very extensively used, most of the group-flashing lights shown in the accompanying tables, being designed upon the general lines he introduced. A modification of the system consists in grouping two or more lenses \\ \ 11 r\~ r,~l. ;~j'},yh fie l /;1`•' / 1\\1g1~C` ,~i~ii" /•/1(`.11 ~I 11111 //I / \\\Pe af°, pt, •/ \~•' i rl,.~:ll ;/,/1/' 1 , ' ' 1 1 // /r // , ' i // 4Q Iii { \ 1T r 1 , Ire' ; y, rl and bright rays by the atmosphere. When such distinction has been employed, as in the Wolf Rock apparatus, the red and white beams can be approximately equalized in initial intensity by constructing the lens and prism panels for the red light of larger angle than those for the white beams. Owing to the absorption by Section. the red colouring, the power of a red beam is only 40% of the intensity of the corresponding white light. The corresponding intensity of green light is 25 %. When red or green sectors are employed they should invariably be reinforced by mirrors, azimuthal condensing prisms, or other means to raise the coloured beam to approximately the same intensity as the white light. With the introduction of group-flashing characteristics the necessity for using colour as a means of distinction disappeared. High-Angle Vertical Lenses.—Messrs Chance of Birmingham have manufactured lenses having 97° of vertical amplitude, but this I. result was only attained by using --- dense flint glass of high refractive revolving light with the panels of reflecting prisms above and below them, setting them at an angle to produce the group-flashing characteristic. The first apparatus of this type constructed were those now in use at Tampico, Mexico and the Little Basses light-house, Ceylon (double flashing). The Casquets apparatus (triple flashing) was installed in 1877. A group-flashing catoptric light had, however, been exhibited from the " Royal Sovereign ' light-vessel in 1875. A sectional plan of the quadruple-flashing first order apparatus together separated by equal angles, and filling the remaining angle in azimuth by a reinforcing mirror or screen. A group-flashing distinction was proposed for gas lights by J. R. Wigham of Dublin, who obtained it in the case of a revolving apparatus by alternately raising and lowering the flame. The first apparatus in which this method was employed was erected at Galley Head, Co. Cork (1878). At this lighthouse 4 of Wigham's large gas burners with four tiers of first-order revolving lenses, eight in each tier, were adopted. By successive lowering and raising of the gas flame at the focus of each tier of lenses he produced the group-flashing distinction. The light showed, instead of one prolonged flash at intervals of one minute, as would be produced by the apparatus in the absence of a gas occulter, a group of short flashes varying in number between six and seven. The uncertainty, however, in the number of flashes contained in each group is found to be an objection to the arrangement. This device was adopted at other gas-illuminated stations in Ireland at subsequent dates. The quadriform apparatus and gas installation at Galley Head were superseded in 1907 by a first order biform apparatus with incandescent oil vapour burner showing five flashes every 20 seconds. Flashing Lights indicating Numbers.—Captain F. A. Mahan, late engineer secretary to the United States Lighthouse Board, devised for that service a system of flashing lights to indicate certain numbers. The apparatus in-stalled at Minot's Ledge lighthouse near Boston Harbour, Massachusetts, has a flash indicating the number 143, thus: the dashes indicating short flashes. Each group is separated by a longer period of darkness than that between succes- FIG. 39.—Pendeen Apparatus. sive members of Plan at Focal Plane. a group. The flashes in a group indicating a figure are about 11 seconds apart, the groups being 3 seconds apart, an interval of 16 seconds' darkness occurring between each repetition. Thus the number is repeated every half minute. Two examples of this system were exhibited by the United States Lighthouse Board at the Chicago Exhibition in 1893, viz, the second-order apparatus just mentioned and a similar light of the first order for Cape Charles on the Virginian coast. The lenses are arranged in a somewhat - --' f index for t h e — upper and lower elements. It is doubtful, how-ever, whether the use of refracting elements for a greater angle than 8o° vertically is attended by any material corresponding advantage. Group Flashing Lights,—One of the most useful distinctions consists in the grouping of two or more flashes separated by short intervals of darkness, the group being succeeded by a longer eclipse. Thus two, three or more flashes of, say, half second duration or less follow each other at intervals of about 2 seconds and are succeeded by an eclipse of, say, to seconds, the sequence being completed in a period of, say, 15 seconds. In 1874 Dr John Hopkinson introduced the very valuable improvement of dividing the lenses of a dioptric c Jrectimenz.line AB E> n similar manner to an ordinary group-flashing light, the groups of lenses being placed on one side of the optic, while the other is provided with a catadioptric mirror. This system of numerical flashing for lighthouses has been frequently proposed in various forms, notably by Lord Kelvin. The installation of the lights described is, however, the first practical application of the system to large and important coast lights. The great cost involved in the alteration of the lights of any country to comply with the requirements of a numerical system is one of the objections to its general adoption. Hyper-radial Apparatus.—In 1885 Messrs Barbier of Paris constructed the first hyper-radial apparatus (1330 mm. focal distance) to the design of Messrs D. and C. Stevenson. This had a height of 1812 mm. It was tested during the South Foreland experiments in comparison with other lenses, and found to give excellent results with burners of large focal diameter. Apparatus of similar focal distance (1330 mm.) were subsequently established at Round Island, Bishop Rock, and Spurn Point in England, Fair Isle and Sule Skerry (fig. 40) in Scotland, Bull Rock and Tory Island in Ireland, Cape d'Antifer in France, Pei Yu-shan in China and a light- house in Brazil. The light erected in 1907 at Cape Race, Newfoundland, is a fine example of a four-sided hyper-radial apparatus mounted on a mercury `. float. The total weight of the revolving part of the light amounts to 7 tons, while the motive clock weight required to rotate this large mass at a speed of two complete revolutions a minute is only 8 cwt. and the weight of mercury required for flotation 950 lb. A similar apparatus was placed at Manora Point, Karachi, India, in 1908 (fig. 41). The introduction of incandescent and other burners of focal compactness and high intensity has rendered the use of optics of such large dimensions as the above, intended for burners of great focal diameter, unnecessary. It is now possible to obtain with a second-order optic (or one of 700 mm. focal distance), having a powerful incandescent petroleum burner in focus, a beam of equal Intensity to that which would be obtained from the apparatus having a 10-wick oil burner or 108-jet gas burner at its focus. Stephenson's Spherical Lenses and Equiangular Prisms.—Mr C. A. Stephenson in 1888 designed a form of lens spherical in the horizontal and vertical sections. This admitted of the construction of lenses of long focal distance without the otherwise corresponding necessity of increased diameter of lantern. A lens of this type and of 1330 mm. focal distance was constructed in 1890 for Fair Isle lighthouse. The spherical form loses in efficiency if carried beyond an angle subtending 200 at the focus, and to obviate this loss Mr Stephenson designed his equiangular prisms, which have an inclination out-wards. It is claimed by the designer that the use of equiangular prisms results in less loss of light and less divergence than is the case when either the spherical or Fresnel form is adopted. An example of this design is seen (fig. 40) in the Sule Skerry apparatus (1895). Fixed and Flashing Lights.—The use of these.lights, which show a fixed beam varied at intervals by more powerful flashes, is not to 9e recommended, though a large number were constructed in the earlier years of dioptric illumination and many are still in existence The distinction can be produced in one or other of three ways: (a) by the revolution of detached panels of straight condensing lens prisms placed vertically around a fixed light optic, (b) by utilizing (evolving lens panels in the middle portion of the optic to produce the flashing light, the upper and lower sections of the apparatus being fixed zones of catadioptric or reflecting elements emitting a fixed belt of light, and (c) by interposing panels of fixed light section between the flashing light panels of a revolving apparatus. In certain conditions of the atmosphere it is possible for the fixed light of low power to be entirely obscured while the flashes are visible, thus vitiating the true characteristic of the light. Cases have frequently occurred of such lights being mistaken for, and even described in lists of light as, revolving or flashing lights. " Cute " and Screens.—Screens of coloured glass, intended to distinguish the light in particular azimuths, and of sheet iron, when it is desired to " cut off " the light sharply on any angle, should befixed as far from the centre of the light as possible in order to reduce the escape of light rays due to divergence. These screens are usually attached to the lantern framing. Divergence.—A dioptric apparatus designed to bend all incident rays of light from the light source in a horizontal direction would, if the flame could be a point, have the effect of projecting a horizontal band or zone of light, in the case of a fixed apparatus, and a cylinder of light rays, in the case of a flashing light, towards the horizon. Thus the mariner in the near distance would receive no light, the rays, visible only at or near the horizon, passing above the level of his eye. In practice this does not occur, sufficient natural divergence being produced ordinarily owing to the magnitude of the flame. Where the electric arc is employed it is often necessary to design the prisms so as to produce artificial divergence. The measure of the natural divergence for any point of the lens is the angle whose sine is the ratio of the diameter of the flame to the distance of the point from centre of flame. In the case of vertical divergence the mean height of the flame must be substituted for the diameter. The angle thus obtained is the total divergence, that is, the sum of the angles above and below the horizontal plane or to right and left of the medial section. In fixed dioptric lights there is, of course, no divergence in the horizontal plane. In flashing lights the horizontal divergence is a matter of considerable importance, determining as it does the duration or length of time the flash is visible to the mariner. Feux-Eclairs or Quick Flashing Lights.—One of the most important developments in the character of lighthouse illuminating apparatus that has occurred in recent years has been in the direction of reducing the length of flash. The initiative in this matter was taken by the French lighthouse authorities, and in France alone forty lights of this type were established between 1892 and 1901. The use of short flash lights rapidly spread to other parts of the world. In England the lighthouse at Pendeen (1900) exhibits a quadruple flash every 15 seconds, the flashes being about a second duration (fig. 39), while the bivalve apparatus erected on Lundy Island (1897)shows 2 flashes of 3 second duration in quick succession every 20 seconds. Since 1900 many quick flashing lights have been erected on the coasts of the United Kingdom and in other countries. The early feux-eclairs, designed by the French engineers and others, had usually a flash of nth to 3rd of a second duration. As a result of experiments carried out in France in 1903-1904, A second has been adopted by the French authorities as the minimum duration for white flashing lights. If shorter flashes are used it is found that the reduction in duration is attended by a corresponding, but not proportionate, diminution in effective intensity. In the case of many electric flashing lights the duration is of necessity reduced, but the greater initial intensity of the flash permits this loss without serious detriment to efficiency. Red or green requires a considerably greater duration than do white flashes. The intervals between the flashes in lights of this character are also small, 2l seconds to 7 seconds. In group-flashing lights the intervals between the flashes are about 2 seconds or even less, with periods of 7 to 10 or 15 seconds between the groups. The flashes are arranged in single, double, triple or even quadruple groups, as in the older forms of apparatus. The feu-eclair type of apparatus enables a far higher intensity of flash to be obtained than was previously possible without any corresponding increase in the luminous power of the burner or other source of light. This result depends entirely upon the greater ratio of condensation of light employed, panels of greater angular breadth than was customary in the older forms of apparatus being used with a higher rotatory velocity. It has been urged that short flashes are insufficient for taking bearings, but the utility of a light in this respect does not seem to depend so much upon the actual length of the flash as upon its frequent recurrence at short intervals. At the Paris Exhibition of 1900 was exhibited a fifth-order flashing light giving short flashes at 1 second intervals; this represents the extreme to which the movement towards he reduction of the period of flashing lights has yet been carried. Mercury Floats.—It has naturally been found impracticable to revolve the optical apparatus of a light with its mountings, some-times weighing over 7 tons, at the high rate of speed required for feux-eclairs by means of the old system of roller carriages, though for some small quick-revolving lights ball bearings have been successfully adopted. It has therefore become almost the universal practice to carry the rotating portions of the apparatus upon a mercury float. This beautiful application of mercury rotation was the invention of Bourdelles, and is now utilized not only for the high-speed apparatus, but also generally for the few examples of the older type still being constructed. The arrangement consists of an annular cast iron bath or trough of such dimensions that a similar but slightly smaller annular hoat immersed in the bath and surrounded by mercury displaces a volume of the liquid metal whose weight is equal to that of the apparatus supported. Thus a comparatively insignificant quantity of mercury, say 2 cwt., serves to ensure the flotation of a mass of over 3 tons. Certain differences exist between the type of float usually constructed in France and those generally designed by English engineers. In all cases pro-vision is made for lowering the mercury bath or raising the float and apparatus for examination. Examples of mercury floats are shown in figs. 41, 42, 43 and Plate I., figs. 54 an 55. 638 Multiform Apparatus.—In order to double the power to be obtained from a single apparatus at stations where lights of exceptionally high intensity are desired, the expedient of placing one complete lens apparatus above another has some-times been adopted, as at the Bishop Rock (fig. 13), and at the Fastnet light-house in Ireland (Plate I., fig. 54). Triform and quadriform apparatus have also been erected in Ireland; particulars of the Tory Island triform apparatus will be found in table VII. The adoption of the multi-form system involves the use of lanterns of in-creased height. Twin Apparatus.—Another method of doubling the power of a light is by mounting two complete and distinct optics side by side on the same revolving table, as I shown in fig. 43 of the Ile Vierge apparatus. Several such lights have been installed by the French Lighthouse Service. Port Lights.—Small self-contained lanterns and lights are in common use for marking the entrances to harbours and in other similar positions where neither high power nor long range is requisite. Many such lights are unattended in the sense that they do not require the attention of a keeper for days and even weeks together. These are de-scribed in more detail in section 6 of this article. A typical port light consists of a copper or brass lantern containing a lens of the fourth order (25o mm. focal distance) or smaller, and a single wick or 2-wick Argand capillary burner. Duplex burners are also used. The apparatus may exhibit a fixed light or, more usually, an occulting characteristic is produced by the revolution of screens actuated by spring clockwork around the burner. The lantern may be placed at the top of a column, or suspended from the head of a mast. Coal gas and electricity are also used as illuminants for port lights when local supplies are available. The optical apparatus used in connexion with electric light is dexribed below. "Orders" of Apparatus. —Augustin Fresnel divided the dioptric lenses, de-signed by him, into "orders" or sizes depending on their local distance. This division is still used, although two additional " orders," known as " small third order " and "hyper-radial" recoPetively are in ordinary use The following LIGHTHOUSE [OPTICAL APPARATUS 1tr kNXIF,A table gives the principal dimensions of the several sizes in use :— Focal Vertical Angles of Optics. (Ordinary Dimensions.) --- Order. Distance, Holophotal Optics. mm. Dioptric Belt only. Lower Lens. Upper Prisms. Prisms. Hyper-Radial 1330 8o° 21° 57° 48° 1st order . 920 92°,80°,58° 21° 57° 48° 2nd „ 700 8o° 21° 57° 480 3rd ,, 500 8o° 210 57° 48° Small 3rd 375 8o° 210 57° 48° order 4th order . 250 80° 21° 57° 48° 5th „ . . 187 5 800 21° 57° 48° 6th 150 800 21° 57° 48° Lenses of small focal distance are also made for buoy and beacon lights. Light Intensities.—The powers of lighthouse lights in the British Empire are expressed in terms of standard candles or in " light-house units " (one lighthouse unit =1000 standard candles). In France the unit is the " Carcel " -952 standard candle. The powers of burners and optical apparatus, then in use in the United Kingdom, were carefully determined by actual photometric measurement in 1892 by a committee consisting of the engineers of the three general lighthouse boards, and the values so obtained are used as the basis for calculating the intensities of all British lights. It was i Half Elevation FIG. 43.—Tie Vierge Apparatus. found that the intensities determined by photometric measurement were considerably less than the values given by the theoretical calculations formerly employed. A deduction of 20% was made from the mean experimental results obtained to compensate for loss by absorption in the lantern glass, variations in effects obtained by different men in working the burners and in the illuminating quality of oils, &c. The resulting reduced values are termed " service " intensities. As has been explained above, the effect of a dioptric apparatus is to condense the light rays, and the measure of this condensation is the ratio between the vertical divergence and the vertical angle of the optic in the case of fixed lights. In flashing lights the ratio of vertical condensation must be multiplied by the ratio between the horizontal divergence and the horizontal angle of the panel. The loss of light by absorption in passing through the glass and by refraction varies from lo% to 15%. For apparatus containing catadioptric elements a larger deduction must be made. The intensity of the flash emitted from a dioptric apparatus, showing a white light, may be found approximately by the empirical formula I = PCVH/vh, where I =intensity of resultant beam; P= service intensity of flame, V=vertical angle of optic, v=angle of mean vertical divergence, H = horizontal angle of panel, h = angle of mean horizontal divergence, and C =constant varying between •9 and •75 according to the description of apparatus. The factor H/h must be eliminated in the case of fixed lights. Deduction must also be made in the case of coloured lights. It should, however, be pointed out that photometric measurements alone can be relied upon to give accurate values for lighthouse intensities. The values obtained by the use of Allard's formulae, which were largely used before the necessity for actual photometric measurements came to be appreciated, are considerably in excess of the true intensities. Optical Calculations.—The mathematical theory of optical apparatus for lighthouses and formulae for the calculations of profiles will he found in the works of the Stevensons, Chance, Allard, Reynaud, Ribiere and others. Particulars of typical lighthouse apparatus will be found in tables VI. and VII. 4. ILLUMINANTS.—The earliest form of illuminant used for lighthouses was a fire of coal or wood set in a brazier or grate erected on top of the lighthouse tower. Until the end of the 18th and even into the 19th century this primitive illuminant continued to be almost the only one in use. The coal fire at the Isle of May light continued until 18ro and that at St Bees lighthouse !n Cumberland till 1823. Fires are stated to have been used hn the two towers of Nidingen, in the Kattegat, until 1846. Smeaton was the first to use any form of illuminant other than coal fires; he placed within the lantern of his Eddystone light-house a chandelier holding 24 tallow candles each of which weighed * of a lb and emitted a light of 2.8 candle power. The aggregate illuminating power was 67.2 candles and the consumption at the rate of 3.4 lb per hour. Oil.—Oil lamps with flat wicks were used in the Liverpool light-houses as early as 1763. Argand, between 178o and 1783, perfected his cylindrical wick lamp which provides a central current of air through the burner, thus allowing the more perfect combustion of the gas issuing from the wick. The contraction in the diameter of the glass chimney used with wick lamps is due to Lange, and the principle of the multiple wick burner was devised by Count Rumford. Fresnel produced burners having two, three and four concentric wicks. Sperm oil, costing 5s. to 8s. per gallon, was used in English lighthouses until 1846, but about that year colza oil was employed generally at a cost of 2s. 9d. per gallon. Olive oil, lard oil and coconut oil have also been used for lighthouse purposes in various parts of the world. • Mineral Oil Burners.—The introduction of mineral oil, costing a mere fraction of the expensive animal and vegetable oils, revolutionized the illumination of lighthouses. It was not until 1868 that a burner was devised which successfully consumed hydro-carbon oils. This was a multiple wick burner invented by Captain Doty. The invention was quickly taken advantage of by lighthouse authorities, and the " Doty " burner, and other patterns involving the same principle, remained practically the only oil burners in lighthouse use until the last few years of the 19th century. The lamps used for supplying oil to the burner are of two general types, viz. these in which the oil is maintained under pressure by mechanical action and constant level lamps. In the case of single wick, and some 2-wick burners, oil is supplied to the burner by the capillary action of the wick alone. The mineral oils ordinarily in use are petroleum, which for lighthouse purposes should have a specific gravity of from •82o to •83o at 6o° F. and flashing point of not less than 230° F. (Abel close test), and Scottish shale oil or paraffin with a specific gravity of about .810 at 6o° F. and flash point of 140° to 165° F. Both these varieties may be obtained in England at a cost of about 61d. per gallon in bulk. Coal Gas had been introduced in 1837 at the inner pier light of Troon (Ayrshire) and in 1847 it was in use at the Heugh lighthouse (West Hartlepool). In 1878 cannel coal gas was adopted for the Galley Head lighthouse, with 1o8-jet Wigharn burners. Sir James Douglass introduced gas burners consisting of concentric rings, two to ten in number, perforated on the upper edges. These give excellent results and high intensity, 2600 candles in the case of the 10-ring burner with a flame diameter at the focal plane of 51 in. They are still in use at certain stations. The use of multiple ring and jet gas burners is not being further extended. Gas for light-house purposes generally requires to be specially made; the erection of gas works at the station is thus necessitated and a considerable outlay entailed which is avoided by the use of oil as an illuminant. Incandescent Coal Gas Burners.—The invention of the Welsbach mantle placed at the disposal of the lighthouse authorities the means of producing a light of high intensity combined with great focal compactness. For lighthouse purposes other gaseous illuminants than coal gas are as a rule more convenient and economical, and give better results with incandescent mantles. Mantles have, however, been used with ordinary coal gas in many instances where a local supply is available. Incandescent Mineral Oil Burners Incandescent lighting with high-flash mineral oil was first introduced by the French Lighthouse Service in 1898 at L'tle Penfret lighthouse. The burners employed are all made on the same principle, but differ slightly in details according to the type of lighting apparatus for which they are intended. The principle consists in injecting the liquid petroleum in the form of spray mixed with air into a vaporizer heated by the mantle flame or by a subsidiary heating burner. A small reservoir of compressed air is used charged by means of a hand pump—for providing the necessary pressure for injection. On first ignition the vaporizer is heated by a spirit flame to the required temperature. A reservoir air pressure of 125 lb per sq. in. is employed, a reducing valve supplying air to the oil at from 6o to 65 lb per sq. in. Small reservoirs containing liquefied carbon dioxide have also been employed for supplying the requisite pressure to the oil vessel. The candle-power of appar- atus in which ordinary multiple Ntpp wick burners were formerly employed is increased by over 300% by the substitution of suitable incandescent oil burners. In 1902 incandescent oil burners were adopted by the general lighthouse authorities in the United Kingdom. The burners used in the Trinity FIG. 44 Chance” Incandescent House Service and some of those made in France have Oil Burner, with 85 mm. diameter the vaporizers placed over the mantle. flame. In other forms, of which the " Chance " burner (fig. 44) is a type, the vaporization is effected by means of a subsidiary burner placed under the main flame. Particulars of the sizes of burner in ordinary use are given in the following table. Diameter of Mantle. Service Intensity. Consumption of oil. Pints per hour. 35 min. 60o candles. .50 55 mm. 1200 „ Poo 85 mm. 2150 2'25 Triple mantle 5o mm. 3300 3.00, The intrinsic brightness of incandescent burners generally may be an aperture in the lantern floor on to another series of prisms, which taken as being equivalent to from 30 candles to 40 candles per sq. cm. of the vertical section of the incandescent mantle. In the case of wick burners, the intrinsic brightness varies, ac-cording to the number of wicks and the type of burner from about 3'5 candles to about 12 candles per sq. cm., the value being at its maximum with the larger type of burner. The luminous intensity of a beam from a dioptric apparatus is, ceteris paribus, proportional to the intrinsic brightness of the luminous source of flame, and not of the total luminous intensity. The intrinsic brightness of the flame of oil burners increases only slightly with their focal'diameter, consequently while the consumption of oil increases the efficiency of the burner for a given apparatus decreases. The illuminating power of the condensed beam can only be improved to a slight extent, and, in fact, is occasionally decreased, by increasing the number of wicks in the burner. The same argument applies to the case of multiple ring and multiple jet gas burners which, notwithstanding their large total intensity, have comparatively small intrinsic brightness. The economy of the new system is instanced by the case of the Eddystone bi-form apparatus, which with the concentric 6-wick burner consuming 2500 gals. of oil per annum, gave a total intensity of 79,250 candles. Under the new regime the intensity is 292,000 candles, the oil consumption being practically halved. Incandescent Oil Gas Burners.—It has been mentioned that incandescence with low-pressure coal gas produces flames of comparatively small intrinsic brightness. Coal gas cannot be compressed beyond a small extent without considerable injurious condensation and other accompanying evils. Recourse has therefore been had to compressed oil gas, which is capable of undergoing compression to 10 or 12 atmospheres with little detriment, and can conveniently be stored in portable reservoirs. The burner employed resembles the ordinary Bunsen burner with incandescent mantle, and the rate of consumption of gas is 27.5 cub. in. per hour per candle. A reducing valve is used for supplying the gas to the burner at constant pressure. The burners can be left unattended for considerable periods. The system was first adopted in France, where it is installed at eight lighthouses, among others the Ar'men Rock light, and has been extended to other parts of the world including several stations in Scotland and England. The mantles used in France are of 35 mm. diameter. The 35 mm. mantle gives a candle-power of 400, with an intrinsic brightness of 20 candles per sq. cm. The use of oil gas necessitates the erection of gas works at the lighthouse or its periodical supply in portable reservoirs from a neighbouring station. A complete gas works plant costs about £800. The annual expenditure for gas lighting in France does not exceed £72 per light where works are installed, or £32 where gas is supplied from elsewhere. In the case of petroleum vapour lighting the annual cost of oil amounts to about £26 per station. Acetylene.—The high illuminating power and intrinsic brightness of the flame of acetylene makes it a very suitable illuminant for lighthouses and beacons, providing certain difficulties attending its use can be overcome. At Grangemouth an unattended 21-day beacon has been illuminated by an acetylene flame for some years with considerable success, and a beacon light designed to run unattended for six months was established on Bedout Island in Western Australia in 1910. Acetylene has also been used in the United States, Germany, the Argentine, China, Canada, &c., for lighthouse and beacon illumination. Many buoys and beacons on the German and Dutch coasts have been supplied with oil gas mixed with 20% of acetylene, thereby obtaining an increase of over t00% in illuminating intensity. In France an incandescent burner consuming acetylene gas mixed with air has been installed at the Chassiron lighthouse (1902). The French Lighthouse Service has perfected an incandescent acetylene burner with a 55 mm. mantle having an intensity of over 2000 candle-power, with intrinsic brightness of 6o candles per sq. cm. Electricity.—The first installation of electric light for lighthouse purposes in England took place in 1858 at the South Foreland, where the Trinity House established a temporary plant for experimental purposes. This installation was followed in 1862 by the adoption of the illuminant at the Dungeness lighthouse, where. it remained in service until the year 1874 when oil was substituted for electricity. The earliest of the permanent installations now existing in England is that at Souter Point which was illuminated in 1871. There are in England four important coast lights illuminated by electricity, and one, viz. Isle of May, in Scotland. Of the former St Catherine's, in the Isle of Wight, and the Lizard are the most powerful. Elctricity was substituted as an illuminant for the then existing oil light at St Catherine's in 1888. The optical apparatus consisted of a second-order 16-sided revolving lens, which was transferred to the South Foreland station in 1904, and a new second order (700 mm.) four-sided optic with a vertical angle of 139°, exhibiting a flash of •21 second duration every 5 seconds substituted for it. A fixed holophote is placed inside the optic in the dark or landward arc, and at the focal plane of the lamp. This holophote condenses the rays from the arc falling upon It into a pencil of small angle, which is directed horizontally upon a series of reflecting prisms which again bend the light and throw it downwards through latter direct the rays seaward in the form of a sector of fixed red light at a lower level in the tower. A somewhat similar arrangement exists at Souter Point lighthouse. The apparatus installed at the Lizard in 1903 is similar to that at St Catherine's, but has no arrangement for producing a subsidiary sector light. The flash is of • 13 seconds duration every 3 seconds. The apparatus replaced the two fixed electric lights erected in 1878. The Isle of May lighthouse, at the mouth of the Firth of Forth, was first illuminated by electricity in 1886. The optical apparatus consists of a second-order fixed-light lens with reflecting prisms, and is surrounded by a revolving system of vertical condensing prisms which split up the vertically condensed beam of light into 8 separate beams of 3° in azimuth. The prisms are so arranged that the apparatus, making one complete revolution in the minute, produces a group characteristic of 4 flashes in quick succession every 30 seconds (fig. 45). The fixed light is not of the ordinary Fresnel section, the refracting portion being confined to an angle of to°, and the remainder of the vertical section consisting of reflecting prisms. In France the old south lighthouse at La Heve was lit by electricity in 1863. This installation was followed in 1865 by a similar one at the north lighthouse. In 1910 there were thirteen important coast lights in France illuminated by electricity. In other parts of the world, Macquarie lighthouse, Sydney, was lit by electricity in 1883; Tino, in the gulf of Spezia, in 1885; and Navesink lighthouse, near the entrance to New York Bay, in 1898. Electric apparatus were also installed at the lighthouse at Port Said in 1869, on the opening of the canal ; Odessa in 1871; and at the Rothersand, North Sea, in 1885. There are several other lights in various parts of the world illuminated by this agency. Incandescent electric lighting has been adopted for the illumination of certain light-vessels in the United States, and a few small harbour and port lights, beacons and buoys. Table VI. gives particulars of some of the more important electric lighthouses of the world. Electric Lighthouse Installations in France.—A list of the thirteen lighthouses on the French coast equipped with electric light installations will be found in table VI. It has been already mentioned that the two lighthouses at La Heve were lit by electric light in 1863 and 1865. These installations were followed within a few years by the establishment of electricity as illuminant at Gris-Nez. In 1882 M. Allard, the then director-general of the French Lighthouse Service, prepared a scheme for the electric lighting of the French littoral by means of 46 lights distributed more or less uniformly along the coast-line. All the apparatus were to be of the same general type, the optics consisting of a fixed belt of. 300 mm. focal distance, around the outside of which revolved a system of 24 faces of vertical lenses. These vertical panels condensed the belt of fixed light into beams of 3° amplitude in azimuth, producing flashes of about sec. duration. To illuminate the near sea the vertical divergence of the lower prisms of the fixed belt was artificially increased. These optics are very similar to that in use at the Souter Point lighthouse, Sunderland. The intensities obtained were 120,000 candles in the case of fixed lights and 900,000 candles with flashing lights. As a result of a nautical inquiry held in 1886, at which date the lights of Dunkerque, Calais, Gris-Nez, La Canche, Baleines and Ill ,i,k _Npnyk /~i~N. Qt~g\N `- --poi-- Planier had been lighted, in addition to the old apparatus at La Heve, it was decided to limit the installation of electrical apparatus to important landfall lights—a decision which the Trinity House had already arrived at in the case of the English coast—and to establish new apparatus at six stations only. These were Creac'h d'Ouessant (Ushant), Belle-Ile, La Coubre at the mouth of the river Gironde, Barfleur, Ile d'Yeu and Penmarc'h. At the same time it was deter-mined to increase the powers of the existing electric lights. The scheme as amended in 1886 was completed in 1902.1 All the electrically lit apparatus, in common with other optics established in France since 1893, have been provided with mercury rotation. The most recent electric lights have been constructed in the form of twin apparatus, two complete and distinct optics being mounted side by side upon the same revolving table and with corresponding faces parallel. It is found that a far larger aggregate candle-power is obtained from two lamps with 16 mm. to 23 mm. diameter carbons and currents of 6o to 120 amperes than with carbons and currents of larger dimensions in conjunction with single optics of greater focal distance. A somewhat similar circumstance led to the choice of the twin form for the two very powerful non-electric apparatus at Ile Vierge (figs. 43 and 43A) and Ailly, particulars of which will be seen in table VII. Several of the de Meritens magneto-electric machines of 5.5 K.W., laid down many years ago at French electric lighthouse stations, are still in use. All these machines have five induction coils, which, upon the installation of the twin optics, were separated into two distinct circuits, each consisting of 21 coils. This modification has enabled the old plants to be used with success under the altered conditions of lighting entailed by the use of two lamps. The generators adopted in the French service for use at the later stations differ materially from the old type of de Meritens machine. The Phare d'Eckmuhl (Penmarc'h) installation serves as a type of the more modern machinery. The dynamos are alternating current two-phase machines, and are installed in duplicate. The two lamps are supplied with current from the same machine, the second dynamo being held in reserve. The speed is 810 to 820 revolutions per minute. The lamp generally adopted is a combination of the Serrin and Berjot principles, with certain modifications. Clockwork mechanism with a regulating electro-magnet moves the rods simultaneously and controls the movements of the carbons so that they are displaced at the same rate as they are consumed. It is usual to employ currents of varying power with carbons of corresponding dimensions according to the atmospheric conditions.. In the French service two variations are used in the case of twin apparatus produced by currents of 6o and 120 amperes at 45 volts with carbons 14 mm. and 18 mm. diameter, while in single optic apparatus currents of 25, 5o and too amperes are utilized with carbon of II mm., 16 mm. and 23 mm. diameter. In England fluted carbons of larger diameter are employed with correspondingly increased current. Alternating currents have given the most successful results in all respects. Attempts to utilize continuous current for lighthouse arc lights have, up to the present, met with little success. The cost of a first-class electric lighthouse installation of the most recent type in France, including optical apparatus, lantern, dynamos, engines, air compressor, siren, &c., but not buildings, amounts approximately to L5900. Efficiency of the Electric Light.—In 1883 the lighthouse authorities of Great Britain determined that an exhaustive series of experiments should be carried out at the South Foreland with a view to ascertaining the relative suitability of electricity, gas and oil as lighthouse illuminants. The experiments extended over a period of more than twelve months, and were attended by representatives of the chief lighthouse authorities of the world. The results of the trials tended to show that the rays of oil and gas lights suffered to about equal extent by atmospheric absorption, but that oil had the advantage over gas by reason of its greater economy in cost of maintenance and in initial outlay on installation. The electric light was found to suffer to a much larger extent than either oil or gas light per unit of power by atmospheric absorption, but the infinitely greater total Intensity of the beam obtainable by its use, both by reason of the high luminous intensity of the electric arc and its focal compactness, more than outweighed the higher percentage of loss in fog. The final conclusion of the committee on the relative merits of electricity, gas or oil as lighthouse illuminants is given in the following words: "That for ordinary necessities of lighthouse illumination, mineral oil is the most suitable and economical illuminant, and that for salient headlands, important landfalls, and places where a very powerful light is required electricity offers the greater advantages." g MISCELLANEOUS LIGHTHOUSE EQUIPMENT. Lanterns.—Modern lighthouse lanterns usually consist of a cast iron or steel pedestal, cylindrical in plan, on which is erected the lantern glazing, sur- 1 In 1901 one of the lights decided upon in 1886 and installed in 1888—Creac'h d'Ouessant—was replaced by a still more powerful twin apparatus exhibited at the 1900 Paris Exhibition. Subsequently similar apparatus to that at Creac'h were installed at Gris-Nez, La Canche, Planier, Barfleur, Belle-Ile and La Coubre, and the old Dunkerque optic has been replaced by that removed from Belle-Ile.mounted by a domed roof and ventilator (fig. 41). Adequate ventilation is of great importance, and is provided by means of ventilators in the pedestal and a large ventilating dome or cowl in the roof. The astragals carrying the glazing are of wrought steel or gun-metal. The astragals are frequently arranged helically or diagonally, thus causing a minimum of obstruction to the light rays in any vertical section and affording greater rigidity to the structure. The glazing is usually 1-in. thick plate-glass curved to the radius of the lantern. In situations of great exposure the thickness is increased. Lantern roofs are of sheet steel or copper secured to steel or cast-iron rafter frames. In certain instances it is found necessary to erect a grille or network outside the lantern to prevent the numerous sea birds, attracted by the light, from breaking the glazing by impact. Lanterns vary in diameter from 5 ft. to 16 ft. or more, according to the size of the optical apparatus. For first order apparatus a diameter of 12 ft. or 14 ft. is usual. Lightning Conductors.—The lantern and principal metallic structures in a lighthouse are usually connected to a lightning conductor carried either to a point below low water or terminating in an earth plate embedded in wet ground. Conductors may be of copper tape or copper-wire rope. Rotating Machinery.—Flashing-light apparatus are rotated by clockwork mechanism actuated by weights. The clocks are fitted with speed governors and electric warning apparatus to indicate variation in speed and when rewinding is required. For occulting apparatus either weight clocks or spring clocks are employed. Accommodation for Keepers, &c.—At rock and other isolated stations, accommodation for the keepers is usually provided in the towers. In the case of land lighthouses, dwellings are provided in close proximity to the tower. The service or watch room should be situated immediately under the lantern floor. Oil is usually stored in galvanized steel tanks. A force pump is sometimes used for pumping oil from the storage tanks to a service tank in the watch-room or lantern. 6. UNATTENDED LIGHTS AND BEACONS.—Until recent years no unattended lights were in existence. The introduction of Pintsch's gas system in the early 'seventies provided a means of illumination for beacons and buoys of which large use has been made. Other illuminants are also in use to a considerable extent. Unattended Electric Lights.—In 1884 an iron beacon lighted by an incandescent lamp supplied with current from a secondary battery was erected on a tidal rock near Cadiz. A 28-day clock was arranged for eclipsing the light between sunrise and sunset and automatically cutting off the current at intervals to produce an occulting characteristic. Several small dioptric apparatus illuminated with incandescent electric lamps have been made by the firm of Barbier Benard et Turenne of Paris, and supplied with current from batteries of Daniell cells, with electric clockwork mechanism for occulting the light. These apparatus have been fitted to beacons and buoys, and are generally arranged to automatically switch off the current during the day-time. They run unattended for periods up to two months. Two separate lenses and lamps are usually provided, with lamp changer, only one lamp being in circuit at a time. In the event of failure in the upper lamp of the two the current automatically passes to the lower lamp. Oil-gas Beacons.—In 1881 a beacon automatically lighted by Pintsch's compressed oil gas was erected on the river Clyde, and large numbers of these structures have since been installed in all parts of the world. The gas is contained in an iron or steel reservoir placed within the beacon structure, refilled by means of a flexible hose on the occasions of the periodical visits of the tender. The beacons, which remain illuminated for periods up to three months are charged to 7 atmospheres. Many lights are provided with occulting apparatus actuated by the gas passing from the reservoir to the burner automatically cutting off and turning on the supply. The Garvel beacon (1899) on the Clyde is shown in fig. 46. The burner has 7 jets, and the light is occulting. Since 1907 incandescent mantle burners for oil gas have been largely used for beacon illumination, both for fixed and occulting lights. Acetylene has also been used for the illumination of beacons and other unattended lights. Lindberg Lights.—In 1881–1882 several beacons lighted automatically by volatile petroleum spirit on the Lindberg-Lyth and Lindberg-Trotter systems were established in Sweden. Many lights of this type have subsequently been placed in different parts of the world. The volatile FIG. 46.—Garvel Beacon. spirit lamp burns day and night. Occulta- tions are produced by a screen or series of screens rotated round the light by the ascending current of heated air and gases from the lamp acting upon a horizontal fan. The speed of rotation of the fan cannot be accurately adjusted, and the times of occultation therefore are liable to slight variation. The lights run unattended for periods up to twenty-one days. Benson-Lee Lamps.—An improvement upon the foregoing is the Benson-Lee lamp, in which a similar occulting arrangement is often used, but the illuminant is paraffin consumed in a special burner having carbon-tipped wicks which require no trimming. The flame intensity of the light is greater than that of the burner consuming light spirit. The introduction of paraffin also avoids the danger attending the use of the more volatile spirit. Many of these lights are in use on the Scottish coast. They are also used in other parts of the United Kingdom, and in the United States, Canada and other countries. Permanent Wick Lights.—About 1891 the French Lighthouse Service introduced petroleum lamps consuming ordinary high-flash lighthouse oil, and burning without attention for periods of several months. The burners are of special construction, provided with a very thick wick which is in the first instance treated in such a manner as to cause the formation of a deposit of carbonized tar on its exposed upper surface. This crust prevents further charring of the wick after ignition, the oil becoming vaporized from the under side of the crust. Many fixed, occulting and flashing lights fitted with these burners are established in France and other countries. In the case of the occulting types a revolving screen is placed around the burner and carried upon a miniature mercury float. The rotation is effected by means of a small Gramme motor on a vertical axis, fitted with a speed governor, and supplied with current from a battery of primary cells. The oil reservoir is placed in the upper part of the lantern and connected with the burner by a tube, to which is fitted a constant level regulator for maintaining the burning level of the oil at a fixed height. In the flashing or revolving light types the arrangement is generally similar, the lenses being revolved upon a mercury float which is rotated by the electric motor. The flashing apparatus established at St Marcouf in 1901 has a beam intensity of moo candle-power, and is capable of running unattended for three months. The electric current employed for rotating the apparatus is supplied by four Lalande and Chaperon primary cells, coupled in series, each giving about 0.15 ampere at a voltage of 0.65. The power required to work the apparatus is at the maximum about o.165 ampere at 0.75 volt, the large surplus of power which is provided for the sake of safety being absorbed by a brake or governor connected with the motor. Wigham Beacon Lights.—Wigham introduced an oil lamp for beacon and buoy purposes consisting of a vertical container filled with ordinary mineral oil or paraffin, and carrying a roller immediately under the burner case over which a long flat wick passes. One end of the wick is attached to a float which falls in the container as the oil is consumed, automatically drawing a fresh portion of the wick over the roller. The other end of the wick is attached to a free counterweight which serves to keep it stretched. The oil burns from the convex surface of the wick as it passes over the roller, a fresh portion being constantly passed under the action of the flame. The light is capable of burning without attention for thirty days. These lights are also fitted with occulting screens on the Lindberg system. The candle-power of the flame is small. 7. LIGHT-VESSELS.—The earliest light-vessel placed in English waters was that at the Nore in 1732. The early light-ships were of small size and carried lanterns of primitive construction and small size suspended from the yard-arms. Modern light-vessels are of steel, wood or composite construction. Steel is now generally employed in new ships. The wood and composite ships are sheathed with Muntz metal. The dimensions of English light-vessels vary. The following may be taken as the usual limits: Length 80 ft. to 114 ft. Beam 20 ft. to 24 ft. Depth moulded . . 13 ft. to 15 ft. 6 in. Tonnage . 155 to 280. The larger vessels are employed at outside and exposed stations, the smaller ships being stationed in sheltered positions and in estuaries. The moorings usually consist of 3-ton mushroom anchors and I e open link cables. The lanterns in common use are 8 ft. in diameter, circular in form, with glazing 4 ft. in height. They are annular in plan, surrounding the mast of the vessel upon which they are hoisted for illumination, and are lowered to the deck level during the day. Fixed lanterns mounted on hollow steel masts are now being used in many services, and are gradually displacing the older type. The first English light-vessel so equipped was constructed in 1904. Of the 87 light-vessels in British waters, including unattended light-vessels, eleven are in Ireland and six in Scotland. At the present time there are over 750 light-vessels in service through-out the world. Until about 1895 the illuminating apparatus used in light-vessels was exclusively of catoptric form, usually consisting of 21 in. or 24 in. silvered parabolic reflectors, having 1, 2 or 3-wick mineral oil ourners in focus. The reflectors and lamps are hung in gimbals to preserve the horizontal direction of the beams. The following table gives the intensity of beam obtained by means of a type of reflector in general use: 2r-in. Trinity House Parabolic Reflector Service Intensity of Beam. Burners i wick " Douglass " 2715 candles „ 2 „ (Catoptric) 4004 „ 2 „ (Dioptric) . . 6722 „ 3 . . . 7528 „ In revolving flashing lights two or more reflectors are arranged in parallel in each face. Three, four or more faces or groups of reflectors are arranged around the lantern in which they revolve, and are carried upon a turn-table rotated by clockwork. The intensity of the flashing beam is therefore equivalent to the combined intensities of the beams emitted by the several reflectors in each face. The first light-vessel with revolving light was placed at the Swin Middle at the entrance to the Thames in 1837. Group-flashing characteristics can be produced by special arrangements of the reflectors. Dioptric apparatus is now being introduced in many new vessels, the first to be so fitted in England being that stationed at the Swin Middle in 1905, the apparatus of which is gas illuminated and gives a flash of 25,000 candle-power. Fog signals, when provided on board light-vessels are generally in the form of reed-horns or sirens, worked by compressed air. The compressors are driven from steam or oil engines. The cost of a modern type of English light-vessel, with power-driven compressed air siren, is approximately £16,000. In the United States service, the more recently constructed vessels have a displacement of 600 tons, each costing £18,000. They are provided with self-propelling power and steam whistle fog signals. The illuminating apparatus is usually in the form of small dioptric lens lanterns suspended at the mast-head—3 or more to each mast, but a few of the ships, built since 1907, are provided with fourth-order revolving dioptric lights in fixed lanterns. There are 53 light-vessels in service on the coasts of the United States with 13 reserve ships. Electrical Illumination.—An experimental installation of the electric light placed on board a Mersey light-vessel in 1886 by the Mersey Docks and Harbour Board proved unsuccessful. The United States Lighthouse Board in 1892 constructed a light-vessel provided with a powerful electric light, and moored her on the Cornfield Point station in Long Island Sound. This vessel was subsequently placed off Sandy Hook (1894) and transferred to the Ambrose Channel Station in 1907. Five other light-vessels in the United States have since been provided with incandescent electric lights—either with fixed or occulting characteristics—including Nantucket Shoals (1896), Fire Island (1897), Diamond Shoals (1898), Overfalls Shoal (1901) and San Francisco (1902). Gas Illumination.—In 1896 the French Lighthouse Service completed the construction of a steel light-vessel (Talais), which was ultimately placed at the mouth of the Gironde. The construction of this vessel was the outcome of experiments carried out with a view to produce an efficient light-vessel at moderate cost, lit by a dioptric flashing light with incandescent oil-gas burner. The construction of the Talais was followed by that of a second and larger, vessel, the Snouw, on similar lines, having a length of 65 ft. 6 in., beam 20 ft. and a draught of 12 ft., with a displacement of 130 tons. The cost of this vessel complete with optical apparatus and gas-holders, with accommodation for three men, was approximately £5000. The vessel was built in 1898-1899.1 A third vessel was constructed in 1901–1902 for the Sandettie Bank on the general lines adopted for the preceding examples of her class, but of the following increased dimensions: length 115 ft.; width at water-line 20 ft. 6 in.; and draught 15 ft., with a displacement of 342 tons (fig. 47). Accommodation is provided for a crew of eight men. The optical apparatus (fig. 48) is dioptric, consisting of 4 panels of 250 mm. focal distance, carried upon a " Cardan " joint below the lens table, and counter-balanced by a heavy pendulum weight. The apparatus is revolved by clockwork and illuminated by compressed oil gas with incandescent mantle. The candle-power of the beam is 35,000. The gas is contained in three reservoirs placed in the hold. The apparatus is contained in a 6-ft. lantern constructed at the head of a tubular mast 2 ft. 6 in. diameter. A powerful siren is provided with steam engine and boiler for working the air compressors. The total cost of the vessel, including fog signal and optical apparatus, was £13,600. A vessel of similar construction to the Talais was placed by the Trinity House in 1905 on the Swin Middle station. The illuminant is oil gas. Gas illuminated light-vessels have also been constructed for the German and Chinese Lighthouse Service. Unattended Light-vessels.—In 1881 an unattended light-vessel, illuminated with Pintsch's oil gas, was constructed for the Clyde, and is still in use at the Garvel Point. The light is occulting, and is shown from a dioptric lens fitted at the head of a braced iron lattice tower 30 ft. above water-level. The vessel is of iron, 40 ft. long, 12 ft. beam and 8 ft. deep, and has a storeholder on board containing oil gas under a pressure of six atmospheres capable of maintaining a light for three months. A similar vessel is placed off Calshot Spit in Southampton Water, and several have been constructed for the l Both the Talais and Snouw light-vessels have since been converted into unattended light-vessels. French and other Lighthouse Services. The French boats are pro- side of the rock. The conductor terminated in a large copper plate, vided with deep main and bilge keels similar to those adopted in the and to the cable end was attached a copper mushroom. Weak larger gas illuminated vessels. In 190I a light-vessel 6o ft. in currents were induced in the lighthouse conductor by the main length was placed off the Otter Rock on the west coast of Scotland; current in the cable, and messages received in the tower by the help ' .3Sm. Lonpiudinot Section - - - - ----- ----- it is constructed of steel, 24 ft. beam, 12 ft. deep and draws 9 ft. of water (fig. 49). The focal plane is elevated 25 ft. above the water-line, and the lantern is 6 ft. in diameter. The optical apparatus is of 500 mm. focal distance and hung in gimbals with a pendulum balance and " Cardan " joint as in the Sandettie light-vessel. The illuminant is oil gas, with an occulting characteristic. The store-holder contains Io,5oo cub. ft. of gas at eight atmospheres, sufficient to supply the light for ninety days and nights. A bell is provided, struck by clappers moved by the roll of the vessel. The cost of the vessel complete was £2979. The Northern Lighthouse Commissioners have four similar vessels in service, and others have been stationed in the Hugli estuary, at Bombay, off the Chinese coasts and elsewhere. In 1909 an unattended gas illuminated light-vessel provided with a dioptric flashing apparatus was placed at the Lune Deep in Morecambe Bay. It is also fitted with a fog bell struck automatically by a gas operated mechanism. Electrical Communication of Light-vessels with the Shore.—Experiments were instituted in 1886 at the Sunk light-vessel off the Essex coast with the view to maintaining telephonic communication with the shore by means of a submarine cable 9 m. in length. Great difficulties were experienced in maintaining communication during stormy weather, breakages in the cable being frequent. These difficulties were subsequently partially overcome by the employment of larger vessels and special moorings. Wireless telegraphic installations have now (1910) superseded the cable communications with light-vessels in English waters except in four cases. Seven light-vessels, including the four off the Goodwin Sands, are now fitted for wireless electrical communication with the shore. In addition many pile lighthouses and isolated rock and island stations have been placed in electrical communication with the shore by means of cables or wireless telegraphy. The Fastnet light- house was, in 1894, electrically connected with the shore by means of a non- continuous cable, it being found impossible to maintain a continuous cable in shallow water near the rock owing to the heavy wash of the sea. A copper conductor, carried down from the tower to below low-water mark, was separated from the cable proper, laid on the bed of the sea in a depth of 13 fathoms, by a distance of about too ft. The lighthouse was similarly connected to earth on the opposite of electrical relays. On the completion of the new tower on the Fastnet Rock in 1906 this installation was superseded by a wireless telegraphic installation. 8. DISTRIBUTION AND DISTINCTION OF LIGHTS, &c.—Methods of Distinction.—The following are the various light characteristics which may be exhibited to the mariner: Fixed.—Showing a continuous or steady light. Seldom used in modern lighthouses and generally restricted to small port or harbour lights, A fixed light is liable to be confused with lights of shipping or other shore lights. Flashing.1—Showing a single flash, the duration of darkness always being greater than that of light. This characteristic or that immediately following is generally adopted for important lights. The French authorities have given the name Feux-Eclair to flashing lights of short duration. Group-Flashing.—Showing groups of two or more flashes in quick succession (not necessarily of the same colour) separated by eclipses with a larger interval of darkness between the groups. Fixed and Flashing.—Fixed light varied by a single white or coloured flash, which may be preceded and followed by a short eclipse. This type of light, in consequence of the unequal intensities of the beams, is unreliable, and examples are now seldom installed although many are still in service. Fixed and Group-Flashing.—Similar to the preceding and open to the same objections. Revolving.—This term is still retained in the " Lists of Lights " issued by the Admiralty and some other authorities to denote a light gradually increasing to full effect, then decreasing to eclipse. At short distances and in clear weather a faint continuous light may be observed. There is no essential difference between revolving and flashing lights, the distinction being merely due to the speed of rotation, and the term might well be abandoned as in the United States lighthouse list. Occulting.—A continuous light with, at regular intervals, one sudden and total eclipse, the duration of light always being equal to or greater than that of darkness. This characteristic is usually exhibited by fixed dioptric apparatus fitted with some form of occulting mechanism. Many lights formerly of fixed characteristic have been converted to occulting. 1 For the purposes of the mariner a light is classed as flashing or occulting solely according to the duration of light and darkness and without any reference to the apparatus employed. Thus, an occulting apparatus, in which the period of darkness is greater than that of light, is classed in the Admiralty " List of Lights " as a " flashing " light. Group Occulting.—A continuous light with, at regular intervals, groups of two or more sudden and total eclipses. Alternating.—Lights of different colours (generally red and white) alternately without any intervening eclipse. This characteristic is not to be recommended for reasons which have already been referred to. Many of the permanent and unwatched lights on the coasts of Norway and Sweden are of this description. Colour.—The colours usually adopted for lights are white, red and green. White is to be preferred whenever possible, owing to the great absorption of light by the use of red or green glass screens. Sectors.—Coloured lights are often requisite to distinguish cuts or sectors, and should be shown from fixed or occulting lightcharacteristic of a light should be such that it may be readily deter-mined by a mariner without the necessity of accurately timing the period or duration of flashes. For landfall and other important coast stations flashing dioptric apparatus of the first order (920 mm. focal distance) with powerful burners are required. In countries where the atmosphere is generally clear and fogs are less prevalent than on the coasts of the United Kingdom, second or third order lights suffice for landfalls having regard to the high intensities available by the use of improved illuminants. Secondary coast lights may be of second, third or fourth order of flashing character, and important harbour lights of third or fourth order. Less important harbours and places where considerable range is not required, as in estuaries and narrow seas, may be lighted by flashing lights of fourth order or smaller size. Where sectors are requisite, occulting apparatus should be adopted for the main light : or subsidiary lights, fixed or occulting, may be exhibited from the same tower as the main light but at a lower level. In such cases the vertical distance between the high and the low light must be sufficient to avoid commingling of the two beams at any range at which both lights are visible. Such commingling or blending is due to atmospheric aberration. Range of Lights.—The range of a light depends first on its elevation above sea-level and secondly on its intensity. Most important lights are of sufficient power to render them visible at the full geographical range in clear weather. On the other hand there are many harbour and other lights which do not meet this condition. The distances given in lists of lights from which lights are visible—except in the cases of lights of low power for the reason given above—are usually calculated in nautical miles as seen from a height of 15 ft. above sea-level, the elevation of the lights being taken as above high water. Under certain atmospheric conditions, and especially with the more powerful lights, the glare of the light may be visible considerably beyond the calculated range. Distances in Distances in Heights Geographical Heights Geographical in Feet. or Nautical in Feet. or Nautical Miles. Miles. 5 2.565 110 12.03 IO 3.628 120 12.56 15 4'443 130 13.08 20 5.130 140 13.57 25 5'736 150 14.02 30 6.283 200 16.22 35 6.787 250 18.14 40 7.255 300 19.87 45 7.696 350 21.46 50 8.112 400 22.94 55 8'509 450 24'33 6o 8.886 500 25.65 65 9.249 550 26.90 70 9'598 600 28.10 75 9'935 65o 29.25 8o Io•26 700 30.28 85 10'57 800 32.45 90 so•88 900 34'54 95 11.18 woo 36.28 100 11.47 apparatus and not from flashing apparatus. In marking the passage through a channel, or between sandbanks or other dangers, coloured light sectors are arranged to cover the dangers, white light being shown over the fairway with sufficient margin of safety between the edges of the coloured sectors next the fairway and the dangers. Choice of Characteristic and Description of Apparatus.—In deter- mining the choice of characteristic for a light due regard must be paid to existing lights in the vicinity. No light should be placed on a coast line having a characteristic the same as, or similar to,another in its neighbourhood unless one or more lights of dissimilar characteristic, and at least as high power and range, intervene. In the case of " landfall lights " the characteristic should differ from any other within a range of too m. In narrow seas the distance between lights of similar characteristic may be less. Landfall lights are, in a sense, the most important of all and the most powerful apparatus available should be installed at such stations. The distinctive EXAMPLE: A tower 200 ft. high will be visible 2o•66 nautical miles to an observer, whose eye is elevated 15 ft. above the water; thus, from the table: 15 ft. elevation, distance visible 4.44 nautical miles 200 ,, ,, 16.22 „ 20.66 „ Elevation of Lights.—The elevation of the light above sea-level need not, in the case of landfall lights, exceed 200 ft., which is sufficient to give a range of over 20 nautical miles. One hundred and fifty feet is usually sufficient for coast lights. Lights placed on high headlands are liable to be enveloped in banks of fog at times when at a lower level the atmosphere is comparatively clear (e.g. Beachy Head). No definite rule can, however, be laid down, and local circumstances, such as configuration of the coast line, must be taken into consideration in every case. Choice of Site.—” Landfall " stations should receive first consideration and the choice of location for such a light ought never to be made subservient to the lighting of the approaches to a port. Subsidiary lights are available for the latter purpose. Lights installed to guard shoals, reefs or other dangers should, when practicable, be placed seaward of the danger itself, as it is desirable that seamen should be able to " make " the light with confidence. Sectors marking dangers seaward of the light should not be employed except when the danger is in the near vicinity of the light. Outlying dangers require marking by a light placed on the danger or by a floating light in its vicinity. 9. ILLUMINATED BuoYs.—Gas Buoys. Pintsch's oil gas has been in use for the illumination of buoys since 1878. In 1883 an automatic occulter was perfected, worked by the gas passing from the reservoir to the burner. The lights placed on these buoys burn continuously for three or more months. The buoys and lanterns are made in various forms and sizes. The spar buoy (fig. 50) may be adopted for situations where strong tides or currents pre- vail. Oil gas lights are frequently fitted to Courtenay whistling (fig. 51) and bell buoys. In the ordinary type of gas buoy lantern the burner employed is of the multiple-jet, Argand ring, or incandescent type. Incan- descent mantles have been applied to buoy lights in France with successful results. Since 1906, and more recently the same system of illumination has been adopted in England and other countries. The lenses employed are of cylindrical dioptric fixed-light form, usually too mm. to 300 mm. diameter. Some of the largest types of gas-buoy in use on the French coast have an elevation from water level to the focal plane of over 26 ft. with a beam intensity of more than woo candles. A large gas-buoy with an elevation of 34 ft. to the focal plane was placed at the entrance to the Gironde in 1907. It has an incan- descent burner and exhibits a light of over 1500 candles. Oil gas forms the most trust- worthy and efficient illuminant for buoy pur- poses yet introduced, and the system has been largely adopted by lighthouse and harbour authorities. There are now over 2000 buoys fitted with oil gas apparatus, in addition to 600 beacons, light-vessels and boats. Electric Lit Buoys.—Buoys have been Level fitted with electric light, both fixed and occulting. Six electrically lit spar-buoys were laid down in the Gedney channel, New York lower bay, in 1888. These were illuminated by too candle-power Swan lamps with continuous current supplied by cable from a power station on shore. The wear and tear of the cables caused considerable trouble and expense. In 189 alternating current was introduced. The installation was superseded by gas lit buoys in 1904. Acetylene and Oil Lighted Buoys.—Acetylene has been extensively employed for the lighting of buoys in Canada and in the United States; to a less extent it has also been adopted in other countries. Both the low pressure system, by which the acetylene gas is produced by an automatic generator, and the so-called high pressure system in which purified acetylene is held in solution In a high pressure gasholder filled with asbestos composition saturated with acetone, have been employed for illuminating buoys and beacons. Wigham oil lamps are also used to a limited extent for buoy lighting. Bell Buoys.—One form of clapper actuated by the roll of the buoy (shown in fig. 52) consists of a hardened steel ball placed in a horizontal phosphor-bronze cylinder provided with rubber buffers. Three of these cylinders are arranged around the mouth of the fixed bell, which is struck by the balls rolling backwards and forwards as the buoy moves. Another form of bell mechanism consists of a fixed bell with three or more suspended clappers placed externally which strike the bell when the buoy rolls. io. FoG SIGNALS.—The introduction of coast fog signals is of comparatively recent date. They were, until the middle of the 19th century, practically unknown except so far as a few isolated bells and guns were con- cerned. The increasing demands of navigation, and the application of steam power to the propulsion of ships resulting in an increase of their speed, drew attention to the necessity of providing suitable signals as aids to navigation during fog and mist. In times of fog the mariner can expect no certain assistance from even a sea fog of even moderate density, at a distance of less than a 4 m. from the shore. The careful experiments and scientific research which have been de-voted to the subject of coast fog-signalling have produced much that is useful and valuable to the mariner, but unfortunately the practical results so far have not been so satisfactory as might be desired, owing to (1) the very short range of the most powerful signals yet produced under certain unfavourable acoustic conditions of the atmosphere, (2) the difficulty experienced by the mariner in judging at any time how far the atmospheric conditions are against him in listening for the expected signa and , acoustic conditions the sounds are audible at considerable ranges. On the other hand, 2-ton bells have been inaudible at distances of a few hundred yards. The 1843 United States trials showed that a bell weighing 4000 lb struck by a 45o lb hammer was heard at a distance of 14 M. across a gentle breeze and at over 9 M. against a 10-knot breeze. Bells are frequently used for beacon and buoy signals, and in some cases at isolated rock and other stations where there is insufficient accommodation for sirens and horns, but their use is being gradually discontinued in this country for situations where a Buoy. the most efficient system of coast lighting, since the beams of light from the most powerful electric lighthouse are frequently entirely dispersed and absorbed by the particles of moisture, forming (3) the difficulty in Buoy. locating the position A, Cylinder, 27 ft. H, Air (compressed of a sound signal by 6 in. long. outlet tube to phonic observations. B, Mooring shackle. whistle. Bells and Gongs are C, Rudder. I, Compressed air in- the oldest and, generD, Buoy. let to buoy. ally speaking, the E, Diaphragm. K, Manhole. least efficient forms F, Ball valves. L, Steps. of fog signals. Under G, Air inlet tubes. N, Whistle. very favourable powerful signal is required. Gongs, usually of Chinese manufacture, were formerly in use on board English lightships and are still used to some extent abroad. These are being superseded by more powerful sound instruments. Expldsive Signals.—Guns were long used at many lighthouse and light-vessel stations in England, and are still in use in Ireland and at some foreign stations. These are being gradually displaced by other explosive or compressed air signals. No explosive signals are in use on the coasts of the United States. In 1878 sound rockets charged with gun-cotton were first used at Flamborough Head and were afterwards supplied to many other stations.' The nitrated gun-cotton or tonite signals now in general use are made up in 4 oz. charges. These are hung at the end of an iron jib or pole attached to the lighthouse lantern or other structure, and fired by means of a detonator and electric battery. The discharge may take place within 12 ft. of a structure without danger. The cartridges are stored for a considerable period without deterioration and with safety. This form of signal is now very generally adopted for rock and other stations in Great Britain, Canada, Newfoundland, northern Europe and other parts of the world. An example will be noticed in the illustration of the Bishop Rock lighthouse, attached to the lantern (fig. 13). Automatic hoisting and firing appliances are also in use. Whistles.—Whistles, whether sounded by air or steam, are not used in Great Britain, except in two instances of harbour signals under local control. It has been objected that their sound has too great a resemblance to steamers' whistles, and they are wasteful of power. In the United States and Canada they are largely used. The whistle usually employed consists of a metallic dome or bell against which the high-pressure steam impinges. Rapid vibrations are set up both in the metal of the bell and in the internal air, producing a shrill note. The Courtenay buoy whistle, already referred to, is an American invention and finds favour in the United States, France, Germany and elsewhere. Reed-Horns.—These instruments in their original form were the invention of C. L. Daboll, an experimental horn of his manufacture being tried in 1851 by the United States Lighthouse Board. In 1862 the Trinity House adopted the instrument for seven land and light-vessel stations. For compressing air for the reed-horns as well as sirens, caloric, steam, gas and oil engines have been variously used, according to local circumstances. The reed-horn was improved by Professor Holmes, and many examples from his designs are now in use in England and America. At the Trinity House experiments with fog signals at St Catherine's (1901) several types of reed-horn were experimented with. The Trinity House service horn uses air at 15 lb pressure with a consumption of .67 cub. ft. per second and 397 vibrations. A small manual horn of the Trinity House type consumes .67 cub. ft. of air at 5 lb pressure. The trumpets of the latter are of brass. Sirens.—The most powerful and efficient of all compressed air fog signals is the siren. The principle of this instrument may be briefly explained as follows:—It is well known that if the tympanic membrane is struck periodically and with sufficient rapidity by air impulses or waves a musical sound is produced. Robinson was the first to construct an instrument by which successive puffs of air under pressure were ejected from the mouth of a pipe. He obtained this effect by using a stop-cock revolving at high speed in such a manner that 720 pulsations per second were produced by the intermittent escape of air through the valves or ports, a smooth musical note being given. Cagniard de la Tour first gave such an instrument the name of siren, and constructed it in the form of an air chamber with perforated lid or cover, the perforations being successively closed and opened by means of a similarly perforated disk fitted to the cover and revolving at high speed. The perforations being cut at an angle, the disk was self-rotated by the oblique pressure of the air in escaping through the slots. H. W. Dove and Helmholtz introduced many improvements, and Brown of New York patented, about 187o, a steam siren with two disks having radial perforations or slots. The cylindrical form of the siren now generally adopted is due to Slight, who used two concentric cylinders, one revolving within the other, the sides being perforated with vertical slots. To him is also due the centrifugal governor largely used to regulate the speed of rotation of the siren. Over the siren mouth is placed a ' The Flamborough Head rocket was superseded by a siren fog signal in 1908.conical trumpet to collect and direct the sound in the desired direction. In the English service these trumpets are generally of considerable length and placed vertically, with bent top and bell mouth. Those at St Catherine's are of cast-iron with copper bell mouth, and have a total axial length of 22 ft. They are 5 in. in diameter at the siren mouth, I the bell mouth being 6 ft. in diameter. At St Catherine's the sirens are two in number, 5 in. in diameter, being sounded simultaneously and in unison (fig. 53). Each siren is provided with ports for producing a high note as well as a low note, the two notes being sounded in quick succession once every minute. The trumpet mouths are separated by an angle of 120° between their axes. This double form has been adopted in certain instances where the angle desired to be covered by the sound is comparatively wide. In Scotland the cylindrical form is used generally, either automatically or motor driven. By the latter means the admission of air to the siren can be delayed until the cylinder is rotating at full speed, and a much sharper sound is produced than in the case of the automatic type. The Scottish trumpets are frequently constructed so that the greater portion of the length is horizontal. The Girdleness trumpet has an axial length of 16 ft., II ft. 6 in. being horizontal. The trumpet is capable of being rotated through an angle as well as dipped below the horizon. It is of cast-iron, no bell mouth is used, and the conical mouth is 4 ft. in diameter. In France the sirens are cylindrical and very similar to the English self-driven type. The trumpets have a short axial length, 4 ft. 6 in., and are of brass, with bent bell mouth. The Trinity House has in recent years reintroduced the use of disk sirens, with which experiments are still being carried out both in the United Kingdom and abroad. For light-vessels and rock stations where it is desired to distribute the sound equally in all directions the mushroom-head trumpet is occasionally used. The Casquets trumpet of this type is 22 ft. in length, of cast-iron, with a mushroom top 6 ft. in diameter. In cases where neither the mush-room trumpet nor the twin siren is used the single bent trumpet is arranged to rotate through a considerable angle. Table IV. gives particulars of a few typical sirens of the most recent form. Since the first trial of the siren at the South Foreland in 1873 a very large number of these instruments have been established both at lighthouse stations and on board light-vessels. In all cases in Great Britain and France they are now supplied with air compressed by steam or other mechanical power. In the United States and some other countries steam, as well as compressed air, sirens are in use. Diaphones.—The diaphone is a modification of the siren, which has been largely used in Canada since 1903 in place of the siren. I t is claimed that the instrument emits a note of more constant pitch than does the siren. The distinction between the two instruments is that in the siren a revolving drum or disk alternately opens and closes elongated air apertures, while in the diaphone a piston pulsating at high velocity serves to alternately cover and uncover air slots in a cylinder. The St Catherine's Experiments.—Extensive trials were carried out dqring 1901 by the Trinity House at St Catherine's lighthouse, Isle of Wight, with several types of sirens and reed-horns. Experiments Plan Double-noted Siren. Station. Description. Vibrations Sounding Cub. ft. of air ft Pressure used per sec. of Remarks. blast reduced per sec. in lb per to atmospheric sq. in. pressure. High. Low. High. Low. St Catherine's (Trinity Two 5-in. cylindrical, 295 182 25 32 i6 The air consump- House) automatically driven tion is for 2 sirens. Girdleness (N.L.C.) sirens 234 100 30 130 26 7-in. cylindrical siren, Casquets (Trinity motor driven .. 98 25 .. 36 7-in. disk siren, motor House) driven 326 .. 28 14 . . A uniform note of French pattern siren . 6-in, cylindrical siren, automatically driven 326 vibrations per sec. has now been adopted generally in France. were also made with different pattern of trumpets, including forms having elliptical sections, the long axis being placed vertically. The conclusions of the committee may be briefly summarized as follows: (I) When a large arc requires to be guarded two fixed trumpets suitably placed are more effective than one large trumpet capable of being rotated. (2) When the arc to be guarded is larger than that effectively covered by two trumpets, the mushroom-head trumpet is a satisfactory instrument for the purpose. (3) A siren rotated by a separate motor yields better results than when self-driven. (4) No advantage commensurate with the additional power required is obtained by the use of air at a higher pressure than 25 lb per sq. in. (5) The number of vibrations per second produced by the siren or reed should be in unison with the proper note of the associated trumpet. (6) When two notes of different pitch are employed the difference between these should, if possible, be an octave. (7) For calm weather a low note is more suitable than a high note, but when sounding against the wind and with a rough and noisy sea a high note has the greater range. (8) From causes which cannot be determined at the time or predicted beforehand, areas sometimes exist in which the sounds of fog signals may be greatly enfeebled or even lost altogether. This effect was more frequently observed during comparatively calm weather and at no great distance from the signal station. (It has often been observed that the sound of a signal may be entirely lost within a short distance of the source, while heard distinctly at a greater distance and at the same time.) (9) The siren was the most effective signal experimented with; the reed-horn, although inferior in power, is suitable for situations of secondary importance. (No explosive signals were under trial during the experiments.) (to) A fog signal, owing to the uncertainty attending its audibility, must be regarded only as an auxiliary aid to navigation which cannot at all times be relied upon. Submarine Bell Signals.—As early as 1841 J. D. Colladon con-ducted experiments on the lake of Geneva to test the suitability of water as a medium for transmission of sound signals and was able to convey distinctly audible sounds through water for a distance of over 21 m., but it was not until 1904 that any successful practical application of this means of signalling was made in connexion with light-vessels. There are at present (1910) over 120 submarine bells in service, principally in connexion with light-vessels, off the coasts of the United Kingdom, United States, Canada, Germany, France and other countries. These bells are struck by clappers actuated by pneumatic or electrical mechanism. Other submerged bells have been fitted to buoys and beacon structures, or placed on the sea bed; in the former case the bell is actuated by the motion of the buoy and in others by electric current, transmitted by cable from the shore. In some cases, when submarine bells are associated with gas buoys or beacons, the compressed gas is employed to actuate the bell striking mechanism. To take full advantage of the signals thus provided it is necessary for ships approaching them to be fitted with special receiving mechanism of telephonic character installed below the water line and in contact with the hull plating. The signals are audible by the aid of ear pieces similar to ordinary telephone receivers. Not only can the bell signals be heard at considerable distances—frequently over Io m.—and in all conditions of weather, but the direction of the bell in reference to the moving ship can be determined within narrow limits. The system is likely to be widely extended and many merchant vessels and war ships have been fitted with signal receiving mechanism. The following table (V.) gives the total numbers of fog signals of each class in use on the 1st of January 1910 in certain countries. When two kinds of signal are employed at any one station, one being subsidiary, the latter is omitted from the enumeration. Buoy and unattended beacon bells and whistles are also omitted, but local port and harbour signals not under the immediate jurisdiction of the various lighthouse boards are included, more especially in Great Britain. II. LIGHTHOUSE ADMINISTRATION. The principal countries of the world possess organized and central authorities responsible for the installation and maintenance of coast lights and fog signals, buoys and beacons. United Kingdom.—In England the corporation of Trinity House,or according to its original charter, " The Master Wardens, and Assistants of the Guild Fraternity or Brotherhood of the most glorious and undivided Trinity and of St Clement, in the Parish of Deptford Strond, in the county of Kent," existed in the reign of Henry VII. as a religious house with certain duties connected with pilotage, and was incorporated during the reign of Henry VIII. In 1565 it was given certain rights to maintain beacons, &c., but not until 168o did it own any lighthouses. Since that date it his gradually purchased most of the ancient privately owned lighthouses and has erected many new ones. The act of 1836 gave the corporation control of English coast lights with certain supervisory powers over the numerous local lighting authorities, including the Irish and Scottish Boards. The corporation now consists of a Master, Deputy-master, and 22 Elder Brethren (to of whom are honorary), together with an unlimited number of Younger Brethren, who, however, perform no executive duties. In Scotland and the Isle of Man the lights are under the control of the Commissioners of Northern Lighthouses constituted in 1786 and incorporated in 1798. The lighting of the Irish coast is in the hands of the Commissioners of Irish Lights formed in 1867 in succession to the old Dublin Ballast Board. The principal local light boards in the United Kingdom are the Mersey Docks and Harbour Board, and the Clyde Lighthouse Trustees. The three general lighthouse boards of the United Kingdom, by the provision of the Mercantile Marine Act of 1854, are subordinate to the Board of Trade, which controls all finances. On the 1st of January 1910 the lights, fog signals and submarine bells in service under the control of the several authorities in the United Kingdom were as follows: Light- Light- Fog marine houses. vessels. Signals. Bells. Trinity House 116 51 97 12 Northern Lighthouse Com- 138 5 44 missioners . Irish Lights Commissioners 93 II 35 3 Mersey Docks and Harbour 16 6 13 2 Board Admiralty 31 2 6 Clyde Lighthouse Trustees 14 I 5 Other local lighting authori- 809 II 89 2 ties Total s . . . . 1217 87 289 19 Some small harbour and river lights of subsidiary character are not included in the above total. United States.—The United States Lighthouse Board was constituted by act of Congress in 1852. The Secretary of Commerce and Labor is the ex-officio president. The board consists of two officers of the navy, two engineer officers of the army, and two civilian scientific members, with two secretaries, one a naval officer, the other an officer of engineers in the army. The members are appointed by the president of the United States. The coast-line of the states, with the lakes and rivers and Porto Rico, is divided into 16 executive districts for purposes of administration. The following table shows the distribution of lighthouses, light-vessels, &c., maintained by the lighthouse board in the United States in June 1909. In addition there are a few small lights and buoys privately maintained. Lighthouses and beacon lights . . 1333 Light-vessels in position . 53 Light-vessels for relief 13 Gas lighted buoys in position . 94 Fog signals operated by steam or oil engines. 228 Fog signals operated by clockwork, &c 205 Submarine signals 43 Post lights . • 2333 Day or unlighted beacons . . 1157 Bell buoys in position . 169 Whistling buoys in position 94 Other buoys . 576o Steam tenders . 51 Constructional Staff . . 318 Light keepers; and light attendants 3137 Officers and crews of light-vessels and tenders 1693 France.—The lighthouse board of France is known as the Commission des Phares, dating from 1792 and remodelled in 1811, and is under the direction of the minister of public works. It consists of four engineers, two naval officers and one member of the Institute, one inspector-general of marine engineers, and one hydrographic engineer. The chief executive officers are an Inspecteur General des Ponts et Chaussees, who is director of the board, and another engineer of the same corps, who is engineer-in-chief and secretary. The board has control of about 750 lights, including those of Horns, o Trumpets, &c. = d N C. M O 5 .- C U u Power. Manual. W ? C7 v F England and Channel Islands 44 , . 27 31 2 15 48 10 16 193 Scotland and Isle of Man 35 6 2 5 .. 16 3 67 Ireland 12 . . 2 6 .. 11 3 11 .. 3 48 France . . . 12 .. 7 I .. 1 .. 25 2 48 United States (excluding in- 43 . . 35 15 59 • • .. 218 I 36 407 land lakes and rivers) . . British North America (ex- 6 66 5 79 16 8 .. 24 .. II 215 cluding inland lakes and rivers) . . Candle- Ratio of Elevation Year o Duration Focal Angular a o0 Electric above - Remarks. E stab- Name. Characteristic. of Flash power Distance Breadth of Generators. Lamps. Engines. High (Service Intensity) of Lens. Panel to u .° U Water, Whole Circle. Whole Standard
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