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night, the two caps of the night-glasses should be opened . On looking through the instrument, any lamp or other light will appear like a fine, bright line, and the range can be taken in the ordinary way . This range-finder possesses the superlative advantage of the one-man instrument, and it is claimed for it that it can range on horizontal objects, such as the crest of a hill, which has no detail suitable for use with a mekometer, and that it can be adjusted on service with no greater difficulty than the setting of a watch . 3 . For harbour defence, owing to the long range of naval guns, and the fast targets which war-vessels present, an accurate range-finder is of first importance . This is largely the case because " ranging " cannot be resorted to in the same manner as in the field, where the targets are comparatively motionless and the effective ranges are less . Successful artillery practice therefore depends, in a great measure, upon the range-finder . The instrument used in harbour forts is known as the depression range-finder . As its name suggests, it solves a triangle in the vertical plane, of which the base is the height of the instrument above sea-level . Its appearance resembles some forms of theodolite (fig . 6) . A framework, capable of rotating in azimuth on a vertical pivot, is supported on a plate carried by levelling screws, L, L, L .

To the framework are pivoted two arms DC and FE, at C and E respectively . The

arm EF is supported at F by a vertical screw H ending in a drum, upon which, in a spiral scale, the ranges are graduated . Motion in altitude is thus given to the telescope . The arm CD is supported by a slider G . This slider is set by a rack and pinion to the height above sea-level (represented on a scale of feet on EF) at which the instrument may be used . A telescope AB is suitably fitted in jaws at the top of the frame . There are spirit-levels at M and Q for adjusting purposes . The telescope is provided with cross wires which can be illuminated for night use . An azimuth circle X and pointer Y enable the direction of any vessel to be indicated, the range of which it is desired to know . The instrument rests on a base plate R, to which it is locked by the top-plate O . The observer directs the cross wires of the telescope upon the water-line of the objective, by means of the drum I and the azimuth handle P, the top of which just appears in the diagram . The reader watches the arrow on the drum and calls out the ranges as the figures arrive beneath it .

The ranges are communicated to the

officers at the guns by various devices, which differ according to local requirements . Position-Finder.—The range-finding instrument known in the British service as the Position-Finder (invented by Colonel Watkin, C.B., R.A.) is practically a large depression range-finder . It posesses, however, certain additional appliances which render it capable of automatically recording, upon an oriented chart, the position or course of a vessel . And further, by electrical means it automatically records to a distant battery the range and bearing of the desired objective . The position-finder can therefore, from a concealed and safe position, coast automatically control the fire of a group of guns, Defence whose detachments need not necessarily see the fnstrn- target engaged . As the observer follows the objec- ments. tive with the telescope of the instrument the range and bearing is simultaneously shown in the battery on convenient dials . The distance and direction thus communicated are the range and bearing from the guns, not as measured from the range-finder . The correction due to the displacement between gun and instrument is automatic . In localities where the height does not admit of using the depression system, an alternative arrangement is provided, known as the Horizontal Position-Finder . It is open to the objections common to two-man range-finders, and is only employed where necessity compels its use . Briefly, there are two observing stations at either end of a measured and electrically connected base . One is known as the transmitting and the other the receiving station; the latter contains the principal instrument, which usually is capable of independent use for medium and short ranges as a depression instrument .

It will be seen that the difference between the two systems is, that the first described solves the range triangle in the vertical, and the latter in the horizontal plane . There have been various methods proposed for using the position-finder . The best results are obtained by placing range and bearing dials on the gun-mounting in a position where they can be easily seen by the men elevating and training the gun . The gun is kept directed upon the objective and fired as quickly as it can be loaded . A position-finder can be used for firing mines in a mine field, and

instruments are issued to the Royal Navy for this purpose . In the United States of America the term " position-finder " is applied to a range-finder which gives direction as well as distance . This is substantially correct, but custom, in the British service, confines the use of the expression as defined above . 4 . Various appliances, not strictly range-finders, are sometimes used to assist in estimating distance . The following examples are not without interest: Acoustic telemeters, depending upon the velocity of sound, are obviously unsuited to the requirements of modern warfare . The names of Thouvenin, Redier and Le Boulenge are connected with such instruments—that of the last-named is perhaps the most convenient . It consists of a graduated glass tube filled with liquid, of suitable density, and containing a small metal traveller .

At the flash of

discharge of a gun or rifle the instrument is brought to a vertical position, and the traveller starts from zero; at the detonation, it is turned to a horizontal position and the traveller stops at the point on the scale indicating the range . On this principle is the rough method of ascertaining the distance, in yards, of a thunderstorm, viz. multiply the number of seconds elapsing between the perception of the lightning and that of the thunder by the number of days in the year . Optical or perspective telemeters determine the distance to any point by observing the size of some object of known dimensions, as seen in a graduated telescope . Porro's telemeter, Elliott's Other telescope and Nordenfelt's macrometer illustrate the prin- Tete- ciple . The chief defect of the system is that the objects meters. most conveniently observed—men and horses—vary con- siderably in size, so that the assumption of a constant dimension may be productive of error . On the continent of Europe the perspective telemeter for military purposes has attracted more attention than in England . The French in their precise terminology call such an instrument " Stadia militaire," a term which at once distinguishes it from a " telemttre," and describes its nature . In rapid military sketching, in locating positions upon maps, &c., perspective telemeters find a use . The telescopes issued .to field batteries and to coast forts in France are provided with a scale in the field of view . By comparing this scale with known heights, such as the average height of a man on foot, or the known height of funnels, masts, turrets, &c., of a war-vessel, distance can be estimated with fair accuracy . The " jumelle Souchier," which can be used as an ordinary field-glass, is constructed on the stadia principle . By its means ranges can be estimated within an accuracy of 1o% .

A stand or

rest, however, is necessary for good results . General Percin of the French army has shown, in an interesting pamphlet, that a piece of wood or card cut to a known fraction of the distance between the eye and the end of the thumb, when the arm is fully extended, can be used to estimate distances . Thus it is easy to find a penny in good condition of which the thickness is -th part of the arm-length in a man of average height . Provided with such a coin an observer finds its rim to exactly cover a distant man 6 ft . (or 2 yds. high) . The range therefore is 400X2 =Soo yds . Similarly, if the man's height appeared to be but half the thickness of the. coin the range would be 4 X400 =1600 yds . With a little practice the eye estimates the proportion between the object of known height and the stadia used . General Percin gives many useful applications of this simple device . Various range-finders have been produced in countries outside the British Isles which, as they are the outcome of similar necessity and required for identical purposes, naturally resemble, more or less, the instruments already described . Field artillery officers of all countries usually claim their gun to be their best range-finder . This may be another way of saying that a durable, one-man range-finder, capable of instantaneously finding modern artillery ranges with accuracy, has yet to be invented .

In France the " telemetre Goutier " for field artillery, a two-man instrument, corresponds with the Watkin mekometer . The "

Gautier," used by the Italian field artillery, is a one-man instrument, but requires a measured base-line . The " Aubry " telemeter, used by some of the Russian batteries in Manchuria, is very portable, but requires a measured base-line, and a slide rule to find the range . In the French and Russian infantry the " prismetelemktre," the invention of Colonel Souchier, is used . It is small, very light, and can be carried in the same manner as field-glasses . French machine guns are ranged by the " telemetre instantane," an instrument of the Barr and Stroud type, with an aluminium base 1 metre in length . For work in the field the modern tendency abroad is to follow Barr and Stroud . In Germany, Hahn, Goerz and Zeiss have produced handy and fairly light short base range-finders, in outward appearance more or less similar to Marindin's instrument . The Zeiss range-finder, however, depends on the stereoscopic principle . It is open to the objection that best results can only be obtained with it by persons who are capable of seeing stereoscopically, and also, in individuals possessing this particular gift (a comparatively small proportion of the human race), stereoscopic vision may vary in power from day to day . Nevertheless the Zeiss range-finder has found favour in many countries, notably as the infantry range-finder in Italy . For naval and harbour defence purposes the Barr and Stroud range-finder is very largely used throughout the world .

In Italy a Barr and Stroud instrument, with the large base of 5 metres, was in 1908 under trial for coast artillery . Of the depression range-finder type in France, " le telemetre Deve " is used at all heights of about 70 ft. and upwards .

Brazil possesses, in the invention of Captain Mario Netto, an excellent range-finder . It is supplied to the harbour defences of that country . It is accurate, handy, easily transported and re-erected where required, and is not affected by the concussion of heavy gun-fire . The German coast range-finder of Hahn closely resembles the earlier Watkin instruments . In Italy the Amici instrument is being replaced by the Braccialine . The latter inventor has also supplied his country with a horizontal base instrument . After extended competitive trials in the U.S.A. the Lewis depression range-finder has been found superior to others presented to the Range-Finding Committee, and is recommended for adoption . It is a neat, workmanlike instrument, and gave an average mean error of 24 yds. in the ranges recorded during the trials . The maximum range was 12,000 yds. and the height of base 1352 ft . The details of position-finders abroad, as in the British service, are confidential, and but little is published of the " telemetre par recoupement " of the French coast batteries, or the " telegoniometro Sollier " of Italy .

In the United States, B . A .

Fiske has ingeniously adapted the principle of the Wheatstone bridge in the construction of the position-finder which bears his name . See de Marre, Instruments pour la mesure de distances (Paris, 188o) ; Abridgments of Specifications, Class 97, Patent Office, London; Handbooks and Instructions for Range-Finder, published by the British War Office; Barr and Stroud, Proc . Inst . Mech . Eng., 30th Jan . 1896; Zeiss pamphlet by Carl Zeiss of Jena, which gives a candid statement of the difficulty attending the stereoscopic principle, &c . (F . M .