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See also:NAVIGATION (from See also:Lat. navis, See also:ship, and agere, to move)
, the See also:science or See also:art of conducting a See also:ship across the seas
.
The See also:term is also popularly used by See also:analogy of boats on See also:rivers, &c., and of flying-See also:machines or similar methods of locomotion
.
See also:Navigation, as an art applied properly to See also:ships, is technically used in the restricted sense dealt with below, and has therefore to be distinguished from " See also:seamanship " (q.v.), or the See also:general methods of See also:rigging a ship (see RIGGING), or the management of sails, See also:rudder, &c
.
See also:History
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The See also:early history of the rise and progress of the art of navigation is very obscure, and it is more easy to trace the See also:gradual advance of See also:geographical knowledge by its means than the growth of the See also:practical methods by which this advance was attained
.
Among Western nations before the introduction of the mariner's See also:compass the only practical means of navigating ships was to keep in sight of See also:land, or occasionally, for See also:short distances, to See also:direct the ship's course by referring it to the See also:sun or stars; this very rough mode of See also:procedure failed in cloudy See also:weather, and even in short voyages in the Mediterranean in such circumstances the navigator generally became hopelessly bewildered as to his position
.
Over the See also:China See also:Sea and See also:Indian Ocean the steadiness in direction of the monsoons was very soon observed, and by See also:running directly before the See also:wind vessels in those localities were able to See also:traverse See also:long distances out of sight of land in opposite directions at different seasons of the See also:year, aided in some cases by a rough compass (q.v.)
.
But it is surprising when we read of the progress made among the ancients in fixing positions on See also:shore by practical See also:astronomy that so many years should have passed without its application to solving exactly the same problems at sea, but this is probably to be explained by the difficulty of devising See also:instruments for use on the unsteady See also:platform of a ship, coupled with the lack of scientific See also:education among those who would have to use them
.
The association of commercial activity and nautical progress shown by the Portuguese in the early See also:part of the 15th See also:century marked an See also:epoch of distinct progress in the methods of practical navigation, and initiated that steady improvement which in the loth century has raised the art of navigation almost to the position of an exact science
.
Up to the See also:time of the Portuguese exploring expeditions, sent out by See also:Prince See also:
See also: Some See also:idea of the speed of See also:ordinary ships in those days may be gathered from an observation in 1551 of a " certain shipp which, without ever striking See also:sail, arrived at See also:Naples from Drepana, in See also:Sicily, in 37 hours " (a distance of 200 M.); the writer accounting for " such See also:swift See also:motion, which to the See also:common sort of man seemeth incredible," by the fact of the occurrence of " violent floods and outrageous winds." In 1578 we find in See also:Bourne's Inventions and Devices a description of a proposed patent log for recording a vessel's speed, the idea (as far as we can gather from its vague description) being to See also:register the revolutions of a See also:wheel enclosed in a See also:case towed astern of a ship (see Loa) . Whether the See also:property of the lodestone was independently discovered in See also:Europe or introduced from the See also:East, it does not appear to have been generally utilized in Europe earlier than about A.D . 1400 (see COMPASS) . In Europe the card or " flie " appears to have been attached to the magnet from the first, and the whole suspended as now in See also:gimbal-rings within the " bittacle," or, as we now spell the word, " See also:binnacle." The direction of a ship's See also:head by compass was termed how she " capes." From the accounts extant of the stores supplied to ships in 1588, they appear to have usually had two compasses, costing 3s . 4d. each, which were kept in See also:charge by the See also:boatswain . The fact that the See also:north point of a compass does not, in most places, point to the true pole but eastward or westward of it, by an amount which is termed by sailors " variation," appears to have been noticed at an early date; but that the amount of variation varied in different localities appears to have been first observed by either Columbus or See also:Cabot about 1490, and we find it used to be the practice to ascertain this See also:error when at sea either from a bearing of the pole star, or by taking a mean of the compass See also:bearings of the sun at both rising and setting, the deviation of the compass in the ships of those days being too small a quantity to be generally noticed, though there is a very suggestive remark on the effect of moving the position of any See also:iron placed near a compass, by a See also:Captain Sturmy of See also:Bristol in 1679 . In order, partially to obviate the error of the compass (variation), the magnets, which usually consisted of two See also:steel wires joined at both ends and opened out in the See also:middle, were not placed under the north and See also:south line of the compass card, but with the ends about a point eastward of north and westward of south, the variation in See also:London when first observed in 1580 being about 11 ° E.; the See also:change of the variation year by year at the same See also:base was first noted by Gellibrand in 1635 . The " cross-staff " appears to have been used by astronomers at a very early See also:period, and subsequently by seamen for measuring altitudes at sea . It was one of the few instruments possessed by Columbus and Vasco da Gama . The old cross-staff, called by the Spaniards " ballestilla," consisted of two See also:light battens . The part we may See also:call the staff was about i; in. square and 36 in. long . The cross was made to See also:fit closely and to slide upon the staff at right angles; its length was a little over 26 in., so as to allow the " pinules or See also:sights to be placed exactly 26 in. apart . A sight was also fixed on the end of the staff for the See also:eye to look through so as to see both those on the cross and the See also:objects whose distance apart was to be measured . It was made by describing the angles on a table, and laying the staff upon it (fig . I) . The See also:scale of degrees was marked on the upper See also:face . Afterwards shorter crosses were introduced, so that smaller angles could be taken by the same instrument . These angles were marked on the sides of the staff . To observe with this instrument a See also:meridian altitude of the sun the bearing was taken by See also:corn-pass, to ascertain when it was near the meridian; then the end of the long staff was placed See also:close to the observer's eye, and the transver- sary, or cross, moved until one end exactly touched the See also:horizon, and the other the sun's centre . This was continued until the sun dipped, when the meridian altitude was obtained . Another See also:primitive instrument in common use at the beginning of the 16th century was the astrolabe (q.v.), which was more See also:con- venient than the cross-staff for taking altitudes . Fig . 2 represents an astrolabe as described by Martin See also:Cortes . It was made of See also:copper or See also:tin, about in. in thickness and 6 or 7 in. in See also:diameter, and was circular except at one See also:place, where a See also:projection was provided for a hole by which it was suspended . See also:Weight was considered desirable in order to keep it steady when in use . The face of the Fetal having been well polished, a plumb line from the point of suspension marked the See also:vertical line, from which were derived the See also:horizontal line and centre . The upper See also:left quadrant was divided into degrees . The second part was a pointer pt of the same See also:metal and thickness as the circular See also:plate, about 11 in. wide, and in length equal to the diameter of the circle . The centre was bored, and a line was drawn across it the full length, which was called the line of confidence . On the ends of that line were fixed plates, s, s, having each a small hole, both exactly over the line of confidence, as sights for the sun or stars . The pointer moved upon a centre the See also:size of a See also:goose See also:quill . When the instrument was See also:sus- pended the pointer was directed by See also:hand to the See also:object, and the See also:angle read on the one quadrant only . Some years later the opposite quadrant was also graduated, to give the benefit of a second See also:reading . The astrolabe was used by Vasco da Gama on his first voyage the See also:movement of a ship rendered accuracy impossible, and the liability to error was increased by the See also:necessity for three observers . One held the instrument by a See also:ring passed over the thumb, the second measured the altitude, and the third read off . For finding latitude at See also:night by altitude of the pole star taken by cross-staff or astrolabe, use was made of an See also:auxiliary instrument called the " nocturnal." From the relative positions of the two stars in the See also:constellation of the " Little See also:Bear " farthest from the pole (known as the Fore and See also:Hind See also:guards) the position of the pole star with regard to the pole could be inferred, and tables were drawn up termed the " See also:Regiment of the Pole Star," showing for eight positions of the guards how much should be added or subtracted from the altitude of the pole star; thus, " when the guards are in the N.W. bearing from each other north and south add See also:half a degree," &c . The bearings of the guards, and also roughly the hour of the night, were found by the nocturnal, first described by M . Coignet in 1581 . The nocturnal (fig . 3) consisted of two concentric circular plates, the See also:outer being about 3 in. in diameter, and divided into twelve equal parts corresponding to the twelve months, each being again sub-divided into See also:groups of five days . The inner circle was graduated into twenty-four equal parts, corresponding to the hours of the day, and again subdivided into quarters; the handle was fixed to the outer circle in such a way that the middle of it corresponded with the day of the See also:month on which the guards had the same right See also:ascension as the sun—or, in other words, crossed the meridian at See also:noon . From the common centre of the two circles extended a long See also:index See also:bar, which, together with the inner circle, turned freely and independently285 about this centre, which was pierced with a See also:round hole . To use the instrument, the projection at twelve hours on the inner plate was turned until it coincided with the day of the month of observation, and the instrument held with its See also:plane roughly parallel to the equinoctial or See also:celestial See also:equator, the observer looking at the pole star through the hole in the centre, and turning the long central index bar until the guards were seen just touching its edge; the hour in line with this edge read off on the.inner plate was, roughly, the time . Occasionally the nocturnal was constructed so as to find the time by observations of the pointers in the See also:Great Bear . The rough charts used by a few of the more See also:expert navigators at the time we refer to will be more fully described later(see also See also:MAP andGEOGRAPUY) . Nautical maps or charts first appeared in See also:Italy at the end of the 13th century, but it is said that the first seen in See also:England was brought by See also:Bartholomew Columbus in 1489 . Among the earliest authors who touched upon navigation was John See also:Werner of See also:Nuremberg, who in 1514, in his notes upon Ptolemy's See also:geography, de-See also:scribes the cross-staff as a very ancient instrument, but says that it was only then beginning to be generally introduced among seamen . He recommends measuring the distance between the See also:moon and a star as a means of ascertaining the longitude; but this (though See also:developed many years after into the method technically known as " lunars ") was at this time of no practical use owing to the then imperfect know-ledge of the true positions of the moon and stars and the non-existence of instrumental means by which such distances could be measured with the necessary accuracy . See also:Thirty-eight years after the discovery of See also:America, when long voyages had become comparatively common, R . Gemma Frisius wrote upon astronomy and See also:cosmogony, with the use of the globes . His book comprised much valuable See also:information to mariners of that day, and was translated into See also:French fifty years later (1582) by See also:Claude de Bossiere . The astronomical See also:system adopted is that of Ptolemy . The following are some of the points of See also:interest See also:relating to navigation . There is a good description of the See also:sphere and its circles; the obliquity of the See also:ecliptic is given as 23° 30' . The distance between the meridians is to be measured on the equator, allowing 15° to an hour of time; longitude is to be found by eclipses of the moon and conjunctions, and reckoned from the Fortunate Islands (Azores) . Latitude should be measured from the equator, not from the ecliptic, " as Clarean says." The use of globes is very thoroughly and correctly explained . The scale for measuring distances was placed on the equator, and 15 See also:German leagues, or 6o See also:Italian leagues, were to be considered equal to one degree . The Italian See also:league was 8 stadia, or loon paces, therefore the degree is taken much too small . We are told that, on plane charts, mariners See also:drew lines from various centres (i.e. compass courses), which were very useful since the virtue of the lodestone had become recognized; it must be remembered that parallel rulers were unknown, being invented by Mordente in 1584 . Such a confusion of lines has been continued upon sea charts till comparatively recently . Gemma gives rules for finding the course and distance correctly, except that he treats difference of longitude as departure . For instance, if the difference of latitude and difference of longitude are equal, the course prescribed is between the two See also:principal winds—that is, 45° . He points out that the courses thus followed are not straight lines, but curves, because they do not follow the great circle, and that distances could be more correctly measured on the globe than on charts . The See also:tide is said to rise with the moon, high water being when it is on the meridian and 12 hours later . From a table of latitudes and longitudes a few examples are here selected, by which it appears that even latitude was much in error . The figures in brackets 2 represent the positions according to See also:modern tables, counting the longitude from the western extremity of St See also:Michael . (See also:Flores is 5° 8' farther See also:west.) See also:Alexandria . 31° 0' N . (31° 13') 6o° 30' E . (550 55') See also:Athens . 37 15 (37 58) 52 45 (49 46) See also:Babylon . 35 0 (32 32) 79 0 (70 25) Dantzic .
54 30 (54 21) 44 15 (44 38)
London
.
52 3 (51 31) 19 15 (25 54)
See also:Malta 34 0 (35 43) 38 45 (40 31)
See also:Rome
.
. 41 50 (41 54) 36 20 (38 30)
The latitude of Cape Clear is given 34' in error, and the longitude 41°; the Scilly Islands are given with an error of one degree in latitude and 1° 1o' in longitude; while See also:Madeira is placed 3° 8' too far south and 40 20' too far west, and Cape St Vincent 1° 25' too far south and 6° too far west
.
In 1534 Gemma produced an
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" astronomical ring," which he dedicated to the secretary of the See also: The upper See also:side is divided into twenty-four parts, repre- senting the hours from noon or midnight . On tae inner side of that circle are marked the months and See also:weeks . The third ring CC is attached to the first at the poles, and revolves freely within it . On the interior are marked the months, and on another side the See also:cor- responding signs of the See also:zodiac; another is gradu- FIG . 4. ated in degrees . It is fitted with a groove which carries two movable sights . On the See also:fourth side are twenty-four unequal divisions (tangents) for measuring heights . Its use is illustrated by twenty problems, showing it capable of doing roughly all that any instrument for taking angles can . Thus, to find the latitude, set the sights C, C to the place of the sun in the zodiac, and shut the circle till it corresponds with 12 o'See also:clock . Look through the sights and alter the point of suspension till the greatest See also:elevation is attained; that time will be noon, and the point of sus-See also:pension will be the latitude . The figure is represented as slung at See also:lat . 400, either north or south . To find the hour of the day, the latitude and declination being known: the sights C, C being set to the declination as before, and the suspension on the latitude, turn the ring CC freely till it points to the sun, when the index opposite the equinoctial circle will indicate the time, while the meridional circle will coincide with the meridian of the place . There is in the museum attached to the Royal See also:Naval See also:College at See also:Greenwich an instrument described as See also:Sir See also:Francis See also:Drake'swatches were unknown till about 1530, when Gemma seized the idea of utilizing them for the purpose of ascertaining the difference of longitude between two places by a comparison between their See also:local times at the same instant . They were too inaccurate, however, to be of practical use, and their See also:advocate proposed to correct them by water-clocks cr See also:sand-clocks . For rough purposes of keeping time on See also:boa:el ship sand glasses were employed, and it is curious to See also:note that hour and half-hour glasses were used for this purpose in the See also:British See also:Navy until 1839 . The outer margin of the compass card was early divided into twenty-four equal parts numbered as hours until the error of thus determining time by the bearings of the sun was pointed out by See also:Davis in 1607 . In 1537 Pedro See also:Nunez (Nonius), cosmographer to the king of Portugal, published a work on astronomy, charts and some points of navigation . He recognized the errors in plane charts, and tried to rectify them . Among many astronomical problems given is one for finding the latitude of a place by knowing the sun's declination and altitude when on two bearings, not less than 400 apart . Gemma did a similar thing with two stars; therefore the problem now known as a " See also:double altitude " is a very old one . It could be mechanically solved on a large globe within a degree . To Nunez has been erroneously attributed the See also:present mode of reading the exact angle on a See also:sextant, t'ha scale of a See also:barometer, &c., the See also:credit of which is due, however. to See also:Vernier nearly a See also:hundred years later . The mode of dividm4 the scale which Nunez published in 1542 was the following . The arc of a large quadrant was furnished with See also:forty-five con-centric segments, or scales, the outer graduated to 9o°, the others to 89, 88, 87, &c., divisions . As the fine edge of the pointer attached to the sights passed among those numerous divisions it touched one of them, suppose the fifteenth See also:division on the See also:sixth scale, then the angle was ii of 90°=15° 52' 56' . This was a laborious method; Tycho See also:Brahe tried it, but abandoned it in favour of the See also:diagonal lines then in common use and still found on all scales of equal parts . In 1545 Pedro de See also:Medina published Arte de navigar at See also:Valla.o dolid, dedicated to See also:Don Philippo, prince of See also:Spain . This appears to be the first book ever published professedly entirely on navigation . It was soon translated into French and Italian, and many years after into See also:English by John Frampton . Though this pretentious work came out two years after the See also:death of See also:Copernicus, the astronomy is still that of Ptolemy . The general See also:appearance of the chart given of the Mediterranean, See also:Atlantic, and part of the Pacific is in its favour, but examination shows it to be very incorrect . A scale of equal parts, near the centre of the chart, extends from the equator to what is intended to represent 75° of latitude; by this scale London would be in 55° instead of 511°, See also:Lisbon in 37}° instead of 38° 42' . The equator is made to pass along the See also:coast of See also:Guinea, instead of being over four degrees farther south . The Gulf of Guinea extends 14° too far east, and See also:Mexico is much too far west . Though there are many vertical lines on the chart at unequal distances they do not represent meridians; and there is no indication of longitude . A scale of 600 leagues is given (German leagues, fifteen to a degree) . By this scale the distance between Lisbon and the See also:city of Mexico is 1740 leagues, or 696o See also:miles; by the vertical scale of degrees it would be about the same; whereas the actual distance is 4820 miles . Here two great wants become apparent—a knowledge of the actual length of any arc, and the means of representing the See also:surface of the globe on See also:flat See also:paper . There is a table of the sun's declination to minutes; on See also:June 12th and See also:December 11th (o.s.) it was given as 23° 33' . The directions for finding the latitude by the pole star and pointers appear good . For general astronomical information the book is inferior to that of Gemma . In 1556 Martin Cortes published at See also:Seville See also:Ark de navigar . He gives a good See also:drawing of the cross-staff and astrolabe, also a table of the sun's declination for four years (the greatest value being 23° 33'), and a See also:calendar of See also:saints' days . The motions of the heavens are described according to the notions then prevalent, the See also:earth being considered as fixed . He recommends astrolabe . It is not an astrolabe, but may be a See also:combination of astronomical rings as invented by Gemma with additions, probably of a later date . It has the appearance of a large See also:gold See also:watch, about 22 in. in diameter, and contains several parts which fall back on hinges . One is a sun-See also:dial, the See also:gnomon being in connexion with a graduated quadrant, by. which it could be set to the latitude of the place . There are a small compass and an hour circle . It is • very neat, but too small for actual use, and may be simply an ornament representing a larger instrument . There is a table of latitudes engraved inside one lid; that given for London is 51° 34', about 3 M. too much . Though clocks are mentioned in 1484 as See also:recent inventions, the altitude of the pole being found frequently, as the estimated distance run was imperfect . He devised an instrument whereby to tell the hour, the direction of the ship's head, and where the sun would set . A very correct table is given of the distances between the meridians at every degree of latitude, whereby a See also:seaman could easily reduce the difference of longitude to departure . In the rules for finding the latitude by the pole star, that star is supposed to be 30 from the pole . Martin Cortes attributes the tides entirely to the See also:influence of the moon, and gives instructions for finding the time of high water at See also:Cadiz, when by means of a card with the moon's See also:age on it, revolving within a circle showing the hours and minutes, the time of high water at any other place for which it was set would be indicated . Directions are given for making a compass similar to those then in common use, also for ascertaining and allowing for the variation . The east is here spoken of as the principal point, and marked by a cross . The third part of Martin Cortes's work is upon charts; he laments that See also:wise men do not produce some that are correct, and that pilots and mariners will use plane charts which are not true . In the Mediterranean and " Channel of See also:Flanders" the want of good charts is (he says) less inconvenient, as they do not navigate by the altitude of the pole . As some subsequent writers have attributed to Cortes the credit of first thinking of the enlargement of the degrees of latitude on See also:Mercator's principle, his precise words may be cited . In making a chart, it is recommended to choose a well-known place near the centre of the intended chart, such as Cape St Vincent, which call 37°, " and from thence towards the See also:Arctic pole the degrees increase; and from thence to the equinoctial line they go on decreasing, and from the line to the See also:Antarctic pole increasing." It would appear at first sight that this implied that the degrees increased in length as well as being called by a higher number, but a specimen chart in the book does not justify that conclusion . It is from 34° to 40°, and the divisions are unequal, but evidently by See also:accident, as the highest and lowest are the longest . He states that the Spanish scale was formed by-counting the Great Berling as 3° from Cape St Vincent (it is under 22°) . Twenty English leagues are equal to 172 Spanish or 25 French, and to 1° of latitude . Cortes was evidently at a loss to know the length of a degree, and consequently the circumference of the globe . The degrees of longitude are not laid down, but for a first meridian we are told to draw a vertical line " through the Azores, or nearer Spain, where the chart is less occupied." It is impossible in such circumstances to understand or check the longitudes assigned to places at that period . Martin Cortes's work was held in high estimation in England for many years, and appeared in several See also:translations . A reprint, with additions, of See also:Richard See also:Eden's (1561), by John Tapp and published in 1609, gives an improved table of the sun's declination from 1609 to 1625—the maximum va,ue being 23° to 3o' . The declinations of the principal stars, the times of their passing the meridian, and other improved tables, are given, with a very poor traverse table for eight points . The cross-staff, he said, was in most common use; but he recommends See also:Wright's sea quadrant .
See also: In 1592 Petrus Plancius published his universal map, containing the discoveries in the East and West Indies and towards the north pole . It possessed no particular merit; the degrees of latitude are equal, but the distances between the meridians are varied . He made London appear in 51° 32' N. and long . 22°, by which his first meridian should have been more than 30 east of St Michael . For Mercator's great improvements in charts at about this date see MAP; from facsimiles of his early charts in Jomard, See also:Les Monuments de la geegraphie, the following measurements have been made . A general chart in 1569 of North America, from lat . 25° to lat . 79°°, is 2 ft. long north and south, and 20 in. wide . Another of the same date, from the equator to 6o° south lat. is 15.8 in. long . The charts agree with each other, a slight See also:allowance being made for remeasuring . As compared with J . See also:Inman's table of meridional parts, the spaces between the parallels are all too small . Between o° and 1o° the error is 8'; at 20° it is 5'; at 3o°, 16'; at 4o°, 39'; at 50 , 61'; at 60°, 104'; at 7o°, 158' ; and at 79°, 182'—that is, over three degrees upon the whole chart . As the See also:measures are always less than the truth it is possible that Mercator was afraid to give the whole . In a chart of Sicily by Romoldus Mercator in 1589, on which two equal degrees of latitude, 36° to 38°, extend 91 in., the degree of longitude is quite correct at one-fourth from the top; the See also:lower part is 1 m. too long . One of the north of See also:Scotland, published in 1595, by Romoldus, measures See also:roe in. from 58° 20' to 61° ; the divisions are quite equal and the lines parallel; it is correct at the centre only . A map of See also:Norway, 1595, lat . 6o° to 700=91i in., has the parallels curved and equidistant, the meridians straight converging lines; the spaces between the meridians at 6o° and 700 are quite correct . In 1594 Blundeville published a description of Mercator's charts and globes; he confesses to not having known upon what rule the meridians were separated by Mercator, unless upon such a table as that given by Wright, whose table of meridional parts is published in the same book, also an excellent table of sines, tangents and secants—the former to seven figures, the latter to eight . These are the tables made originally by Regiomontanus and improved by Clavius . In 1594 the celebrated navigator John Davis published a pamphlet of eighty pages, in See also:black See also:letter, entitled The Seaman's Secrets, in which he proposes to give all that is necessary for sailors—not for scholars on shore . He defines three kinds of sailing: horizontal, paradoxical and great circle . His horizontal sailing consists of short voyages which may be delineated uppn a See also:plain See also:sheet of paper . The paradoxical or cosmographical embraces longitude, latitude and distance—the combining many horizontal courses into one " infallible and true," i.e. what is now called traverse and Mercator's sailings . His " paradoxical course " he describes correctly as a rhumb line which is straight on the chart and a See also:curve on the globe . He points out the errors of the common or plane chart, and promises if spared to publish a " paradoxall chart." It is not known whether such appeared or not, but he assisted Wright in producing his chart on what is known as Mercator's projection a few years later . Great circle sailing on a globe is clearly described by Davis, and to render it more practicable he divides a long distance into several short rhumb lines quite correctly . From the practice of navigators in using globes the principles of such sailing were not unknown at an earlier date; indeed it is said that S . Cabot. projected a voyage across the North Atlantic on the arc of a great circle in 1495 . The See also:list of instruments given by Davis as necessary to a skilful seaman comprises the sea compass, cross-staff, chart, quadrant, astrolabe, an " instrument magnetical " for finding the variation of the compass, a horizontal plane sphere, a globe and a paradoxical compass . The first three are said to be sufficient for use at sea, the astrolabe and quadrant being uncertain for sea observations . The importance of knowing the times of the tides when approaching tidal or barred harbours is clearly pointed out, also the mode of ascertaining them by the moon's age . A table of the sun's declination is given for noon each day during four years 1593-1597, from the ephemerides of J . Stadius . The greatest given value is 23° 28' . Several courses and distances, with the resulting difference of latitude and departure, are correctly worked out . A specimen log-book provides one line only for each day, but the columns are arranged similarly to those of a modern log . Under the head of remarks after leaving See also:Brazil, we read, " the compass varied 9°, the south point westward." He states that the first meridian passed through St Michael, because there was no variation at that place, and therefore that this meridian passed through the magnetic pole as well as the pole of the earth . He makes no mention of Mercator's chart by name nor of Cortes or other writers on navigation . Rules are given for finding the latitude by two altitudes of the sun and intermediate See also:azimuth, also by two fixed stars, using a globe . There is a drawing of a quadrant, with a plumb line, for measuring the See also:zenith distance, and one of a modification of a cross-staff using which the observer stands with his back to the sun, looking at the horizon through a sight on the end of the staff, while the See also:shadow of the top of a movable projection, falls on the sight; this, known as the back-staff, was an improvement on the cross-staff . It was fitted with a reflector, and was thus the first rough idea of the principle of the quadrant and sextant . This remained in common use till superseded in 1731 by See also:Hadley's quad-rant . The eighth edition of Davis's work was printed in 1657 . See also:Edward Wright, of See also:Caius College, See also:Cambridge, published in 1599 a valuable work entitled Certain Errors in Navigation Detected and Corrected . One part is a See also:translation from Roderico Zamorano; there is a See also:chapter from Cortes and one from Nunez . A year later appeared his chart of the See also:world, upon which both capes and the recent discoveries in the East Indies and America are laid down truthfully and scientifically, as well as his know-ledge of their latitudes and longitudes would admit . Just the See also:northern extremity of See also:Australia is shown . Wright said of himself that he had striven beyond his ability to mend the errors in chart, compass, cross-staff and declination of sun and stars . He considered that the instruments which had then recently come in use " could hardly be amended," as they were growing to " perfection "--especially the sea chart and the compass, though he expresses a See also:hope that the latter may be " freed from that rude and See also:gross manner of handling in the making." He gives a table of magnetic declinations (variation) and explains its geometrical construction . He states that Medina utterly denied the existence of variation, and attributed it to See also:bad construction and bad observations . Wright expresses a hope that a right understanding of the See also:dip of the needle would See also:lead to a knowledge of the latitude, " as the variation did of the longitude." He gives a table of declination of the sun for the use of English mariners during four years—the greatest given value being 23° 31' 30" . The latitude of London he made 51° 32' . For these determinations a quadrant over 6 ft. in See also:radius was used . He also treats of the " dip " of the sea horizon, See also:refraction, See also:parallax and the sun's motions . With all this knowledge the earth is still considered as stationary—although Wright alludes to Copernicus, and says that he omitted to allow for parallax . Wright ascertained the declinations of thirty-two stars, and made many improvements or additions to the art of navigation, considering that all the problems could be performed trigonometrically, without globe or chart . He devised sea rings for taking observations, and a sea quadrant to be used by two persons, which is in some respects similar to that by Davis . While deploring the neglected state which navigation had been in, he rejoices that the worshipful society at the Trinity See also:House (which had been established in 1514), under the favour of the king (Henry VIII.), had removed " many gross and dangerous enormities." He joins the brethren of the Trinity House in the desire that a lectureship should be established on navigation, as at Seville and Cadiz; also that a See also:grand See also:pilot should be appointed, as See also:Sebastian Cabot had been in Spain, to examine pilots (i.e. mates) and navigators . Wright's desire was partially fulfilled in 1845, when an Act of See also:Parliament paved the way for the compulsory qualification of masters and mates of See also:merchant ships; but such was the opposition by shipowners that it was even then left voluntary for a few years .
England was in this respect more than a century behind See also: To keep this proportion on the chart, the distances between points of latitude must increase in the same proportion as the secants of the arc contained between those points and the equator, which was then to be done by the " See also:canon of triangles." Wright gave the following excellent popular description of the principle of Mercator's charts: " Suppose a spherical globe (representing the g world) inscribed in a See also:concave See also:cylinder to swell like a See also:bladder equally in every part (that is as FIG . 5. much in longitude as in latitude) until it joins itself to the concave surface of the cylinder, each parallel in-creasing successively from the equator towards either pole until it is of equal diameter to the cylinder, and consequently the meridians widening apart until they are everywhere as distant from each other as they are at the equator . Such a spherical surface is thus by See also:extension made cylindrical, and consequently a plane parallelogram surface, since the surface of a cylinder is nothing else but a plane parallelogram surface See also:wound round it . Such a cylinder on being opened into a flat surface will have upon it a See also:representation of a l\Iercator's chart of the world." This great improvement in the principle of constructing charts was adopted slowly by seamen, who, putting it as they supposed to a practical test, found good See also:reason to be disappointed . The positions of most places in the world had been originally laid down erroneously, by very rough courses and estimated distances upon the plane chart, and from this they were transferred to the new projection, so that errors in courses and distances, really due to erroneous positions, were wrongly attributed to the new and accurate form of chart . When See also:Napier's Canon Mirificus appeared in 1614, Wright at once recognized the value of logarithms as an aid to navigation, and undertook a translation of the book, which he did not live to publish (see NAPIER) . See also:Gunter's tables (162o) made the application of the new discovery to navigation possible, and this was done by See also:Addison in his Arithmetical Navigation (1625), as well as by Gunter in his tables of 1624 and 1636, which gave logarithmic sines and tangents, to a radius of x,000,000, with directions for their use and application to astronomy and navigation, and also logarithms of See also:numbers from r to Io,000 . Several See also:editions followed, and the work retained its reputation over a century . Gunter invented the sector, and introduced the meridional line upon it, in the just proportion of Mercator's projection . The means of taking observations correctly, either at sea or on shore, was about this time greatly assisted by the invention bearing the name of See also:Pierre Vernier, the description of which was published at See also:Brussels in 1631 . As Vernier's quadrant was divided into half degrees only, the sector, as he called it, spread over 141 degrees, and that space carried thirty equal divisions, numbered from o to 30 . As each division of the sector contained 29 min. of arc, the vernier could be read to minutes .
The verniers now commonly adapted to sextants can be read to ro secs
.
Shortly after the invention it was recommended for use by P
.
See also:Bouguer and Jorge Juan, who describe it in a treatise entitled La Construction, &c., du quadrant nouveau
.
About this period See also:Gascoigne applied the See also:telescope to the quadrant as used on shore; and See also:Hevelius invented the tangent See also:screw, to give slow and steady motion when near the desired position
.
These
of the compass and comparing it with that laid down on charts
.
In 1674 See also: 
W . Wargentin, and many other astronomers . But this method, though useful on land, is not suited to mariners; when W . See also:Whiston, for example, in 1737 recommended that the satellites should be observed by a reflecting telescope, he did not sufficiently consider the difficulty of using a telescope at sea . Another method proposed was that of comparing the local time of the moon's See also:crossing the meridian of the observer with the predicted time of the same event at Greenwich, the difference of the two de-pending upon the moon's motion during the time represented by the longitude; thus See also:Herne's Longitude Unveiled (1678), proposes to find the time of the moon's meridian passage at sea by equal altitudes with the cross-staff, and then compare apparent time at ship with London time . The accuracy of this, as in the case of lunar problems, would obviously depend upon a more perfect knowledge of the See also:laws of the moon's motion than then existed . The celebrated problem of finding longitude by lunars (or by measurement of " lunar distances ") occupied the See also:attention of astronomers and sailors for many years before being superseded by the more See also:simple and accurate modern method by the use of chronometers, and was the principal reason for establishing the Royal Observatory at Greenwich and the subsequent publication of the Nautical See also:Almanac . The principle was simple, depending upon the comparatively rapid movement of the moon with regard to the heavenly bodies lying in her immediate path in the heavens . It is evident that if the theory of this movement were perfectly under-stood and the positions of such heavenly bodies accurately deter-See also:mined, the distances of the moon from those at any instant of time at Greenwich could be accurately foretold so that if such predictions were published in advance, an observer at any place in the world; by simply measuring such distances, could accurately determine the Greenwich time, a comparison of which with the local time (which in clear weather can be frequently and simply determined) would give the longitude . This, as previously mentioned, was foreseen by J . Werner as early as 1514, but very great difficulties attended its practical application for many years . Until the See also:establishment of See also:national astronomical observatories it was impossible to accumulate the vast number of observations necessary to fulfil the astronomical conditions, and until the invention of the sextant no instrument existed capable of use at sea which would measure the distances required with the necessary accuracy, while even up to the time when the problem had attained its greatest practical accuracy the calculations involved were far too intricate for general use among those for whom it was chiefly intended .
The very principles of a theory of the movements of the moon were unknown before See also:Newton's time, when the lunar problem begins to have a See also:chief place in the history of navigation; the places of stars were formerly derived from various and widely discrepant See also:sources
.
The study of the lunar problem was stimulated by the See also:reward of woo crowns offered by See also: See also:Borough and See also:Edmund Gunter . The latter was his predecessor at Gresham College . In 1637 Richard See also:Norwood, a sailor, and reader in See also:mathematics, published an account of his most laudable exertions-to remove one of the greatest stumbling-blocks in the way of correct navigation, that of not knowing the true length of a degree or nautical mile, in a pamphlet styled The Seaman's Practices . Norwood ascertained the latitude of a position near the Tower of London in June 1633, and of a place in the centre of See also:York in June 1635, with a sextant of more than 5 ft. radius, and, having carefully corrected the declination of the sun and allowed for refraction and parallax, made the difference of latitude z° 28' . He then measured the distance with a See also:chain, taking horizontal angles of all windings, and made a See also:special table for correcting elevations and depressions . A few places which he was unable to measure he paced . His conclusion was that a degree contained 367,176 English feet; this gives 2 4o yds. to a nautical mile—only about 12 yds. too much . Norwood's work went through numerous editions, and retained its popularity over a hundred years . In a late edition he says that, as there is no means of discovering the longitude, a seaman must See also:trust to his reckoning . He recommends the knots on the log-line to be placed 51 ft. apart, as the just proportion to a mile when used with the half-minute glass . To Norwood is also attributed the discovery of the " dip " of the magnetic needle in 1576 . The progress of the art of navigation was and is still of course inseparably connected with that of map and chart drawing and the correct astronomical determinations of positions on land .
• While as we have seen at an early period simple practical astronorbical means of finding the latitude at sea were known and in use, no mode could be devised of finding longitude except by the rough method of estimating the run of the ship, so that the only mode of arriving at a See also:port of destination was to See also:steer so as to get into the latitude of such a port either to the eastward or west-See also:
This has been accomplished by much labour and See also:patience; for, though originally the most suitable man in the See also:kingdom was placed in charge, it was so starved and neglected as to be almost useless during many years
.
The See also:government did not provide a single instrument
.
Flamsteed entered upon his important duties with an iron sextant of 7 ft. radius, a quadrant of 3 ft. radius, two telescopes and two clocks, the last given by Sir Jonas Moore
.
Tycho Brahe 's See also:catalogue of 777 stars, formed in about 1590, was his only See also:guide
.
In 1681 he fitted a mural arc which proved a failure
.
Seven years after another mural arc was erected at a cost of £120, with which he set to work in See also:earnest to verify the latitude, and to determine the position of the equinoctial point, the obliquity of the ecliptic and the right ascensions and declinations of the stars; he obtained the positions of 2884 which appeared in the " British catalogue " in 1723 (see FLAMSTEED, and ASTRONOMY)
.
Flamsteed died in 1719, and was succeeded by Halley, who paid particular attention to the motions of the moon with a view to the longitude problem
.
A paper which he published in the Phil
.
Trans
.
(1731) shows what had been accomplished up to that date, and proves that it was still impossible to find the longitude correctly by any observation depending upon the predicted position of the moon
.
He repeats what he had published twenty years before in an appendix to See also: He hopes that the tables will be so amended that an error may scarce ever exceed 3 minutes of arc (equal to II° of longitude) . Sir See also:Isaac Newton's tables, corrected by himself (Halley) and others up to 1713, would admit of errors of 5 minutes, when the moon was in the third and fourth quarters . He blames Flamsteed for neglecting that portion of astronomical work, as he was at the observatory more than two periods of eighteen years . He himself had at this time seen the whole period of the moon's apogee—less than nine years—during which he observed the right ascensions at her transit, with great exactness, almost fifteen hundred times, or as often as Tycho Brahe, Hevelius and Flamsteed together . He hoped to be able to compute the moon's position within 2 minutes of arc with certainty, which would reduce errors of position to 20 leagues at the equator and 15 in the Channel; he thought Hadley's quadrant might be applied to measure lunar distances at sea with the desired accuracy.' The rise of modern navigation may be fairly dated from the invention of the sextant in 1731 and of the chronometer in 1735; the former a See also:complete nautical observatory in itself, and the latter an instrument which in its modern development has become an almost perfect time-keeper . It was a curious co-incidence that these two invaluable instruments were invented at so nearly the same time . Until 1731 all instruments in use at sea for measuring angles either depended on a plumb line or required the observer to look in two directions at once . Their imperfections are clearly pointed out in a paper by Pierre Bouguer (1729) which received the See also:prize of the Paris See also:Academy of Sciences for the best method of taking the altitude of stars at sea . Bouguer himself proposes a modification of what he calls the English quadrant, probably the one suggested by Wright and improved by Davis . Fig . 6 represents the instrument as proposed, capable of measuring fully 9o° from E to N . A fixed pinule was recommended to be placed at E, through which a See also:ray from the sun would pass to the sight C . The sight F was movable . The observer, standing with his back to the sun would look through F and C at the horizon, shifting the sight F up or down till the ray from the sun coincided with the horizon . The space from E to F would represent the altitude, and the remaining part F to N the zenith distance . The English quad-rant which this was to supersede differed in having about half the arc from E towards N, and, instead of the pinule being fixed at E, it was on a smaller arc represented by the dotted line eB, and movable . It was placed on an even number of degrees, considerably less than the altitude; the See also:remainder was measured on the larger arc, as described . 1 Halley's observations were published posthumously in 1742, and in 1765 the commissioners of longitude paid his daughter £loo for See also:MSS. supposed to be useful to navigation . As the moon passes the stars lying in her course through the heavens at the mean rate of 33" in one minute of time, it is obvious that an error to that amount in measuring the distance from a star would. produce an error of 15 m. in longitude . As the moon's motion with regard to the sun is nearly one degree a day less, a similar error in the distance would produce still more effect . Hadley's instrument, on the other hand, described to the Royal Society in May 1731 (Phil . Trans.), embodies Newton's idea of bringing the reflection of one object to coincide with the direct See also:image of the other . He calls it an octant, as the arc is actually 450, or the eighth part of a circle; but, in consequence of the angles of incidence and reflection both being changed E by a movement of the index, it measures an angle of 900, and is graduated accordingly; the same instrument has therefore been called a quadrant . It was very slowly adopted, and no doubt there were numerous See also:mechanical difficulties of centring, graduating, &c., to be overcome before it reached perfection .
In See also:August 1732, in pursuance of an order from the See also:Admiralty, observations were made with Hadley's quadrant on board the " See also:Chatham" yacht of 6o tons, below Sheer-
ness, in rough weather, by persons—except the See also:master attendant —unaccustomed to the motion; still the results were very satisfactory
.
A year later Hadley published (Phil
.
Trans., 1733) the description of an instrument for taking altitudes when the horizon is not visible
.
The sketch represents a curved See also:tube or spirit-level, attached to the radius of the quadrant, since which time many attempts have been unsuccessfully made to construct some form of artificial horizon adapted to use at sea on board ship, a discovery which would greatly facilitate observations at night and at the many times when the natural or sea horizon is imperfectly visible
.
From the year 1714 the history of navigation in England is closely associated with that of the " Commissioners for the discovery of longitude at sea," a See also:body constituted in that year with power to See also: Thus, in a memorial by Captain H . Lanoue (1736), he records a number of recent casualties, which shows how carelessly the largest ships were then navigated . Several men-of-See also:war off See also:Plymouth in 1691 were 2 This See also:total comprises the large sums awarded to See also:Harrison and to the widow of See also:Mayer, the cost of surveys and expeditions in various parts of the globe, large outlays on the Nautical Almanac and on subsidiary calculations and tables, rewards for new methods and solutions of problems, and many minor grants to watchmakers or for improvements in instruments . Thus See also:Jesse See also:Ramsden received in 1775 and later about £1600 for his improvements in See also:graduation (q.v.), and E . See also:Massey in 1804 got £200 for his log (see LoG) . wrecked through mistaking the Deadman for See also:Berry Head . See also:Admiral See also:Wheeler's See also:squadron in 1694, leaving the Mediterranean, ran on See also:Gibraltar when they thought they had passed the Strait . Sir Cloudesley See also:Shovel's squadron, in 1707, was lost on the rocks off Scilly, by erring in their latitude . Several transports, in 1711, were lost near the See also:river St See also:Lawrence, having erred 15 leagues in the reckoning during twenty-four hours . See also:Lord Belhaven was lost on the See also:Lizard on the 17th of See also:November 172I, the same day on which he sailed from Plymouth . Many rewards were paid by the commissioners for methods by which the tedious calculations involved in " clearing the lunar distance " could be abbreviated; thus See also:Israel See also:Lyons (1739–1775) received £to for his See also:solution of this problem from the commissioners in 1769; and in 1i72 he and Richard Dunthorne (1711–1775) each obtained X50 . George Whichell, master of the Royal Naval Academy, See also:Portsmouth, conceived a See also:plan whereby the correction could be taken from a table by inspection .
In See also:October 1765 the commissioners of longitude awarded him £too to enable him to complete and See also:print moo copies of his table
.
On the following See also:April they gave him £200 more
.
The work was continued on the same plan by Antony Shepherd, the Plumian See also:professor of astronomy, Cambridge, with some additions by the astronomer-royal
.
The total cost of the ponderous 4to See also:volume up to the time of publication in June 1772 was £3100, after which £200 more was paid to the Rev
.
Thomas See also:Parkinson and Israel Lyons for examining the errata
.
It was a very large and expensive volume—See also:ill-adapted for ship's use
.
Considerable sums were paid by the commissioners from time to time for other tables to facilitate navigation—not always very judiciously
.
It is sufficient to mention here the tables of Michael See also: Indeed it is to them that we owe all that was done by England for surveys of coasts, both at See also:home and abroad, See also:prior to the establishment of the hydrographic See also:department of the Admiralty in 1795 . But their chief work See also:lay in the encouragement they gave on the one hand to the improvement of timepieces, and on the other to the perfecting of astronomical tables and methods, the latter being published from time to time in the Nautical Almanac . Before we pass on to these two important topics we may with See also:advantage take a view of the state of practical navigation in the middle of the 18th century as shown in two of the principal See also:treatises then current . John See also:Robertson's Elements of Navigation passed through six editions between 1755 and 1796 . It contains good teaching on See also:arithmetic, See also:geometry, spherical See also:trigonometry, astronomy, geography, winds and tides, also a small useful table for correcting the middle time between the equal altitudes of the sun—all good, as is also the remark that " the greater the moon's meridian altitude the greater generally the tides will be." He states that See also:Lacaille recommends equal altitudes being observed and worked separately,'in order to find the time from noon, and the mean of the results taken as the truth . There is a See also:sound See also:article on See also:chronology, the ancient and modern modes of reckoning time . A long list of latitudes, longitudes and times of high water finishes vol. i . The second volume is said by the author to treat of navigation mechanical and theoretical ; by the former he means seamanship . He gives instructions for all kinds of railings, for marine See also:surveying and making Mercator's chart . There are two good traverse tables, one to See also:quarter points, the other to every 15 minutes of arc; the distance to each is 120 M . There is a table of meridional parts to minutes, which is more minute than customary . Book ix., upon what is now called " the day's work," or dead-reckoning, appears to embrace all that is necessary .
A great many methods, we are told, were then used for measuring a ship's rate of sailing, but among the English the log and line with a half-minute glass were generally used
.
Bouguer and Lacaille See also:pro-posed a log with a See also:diver to avoid the See also:drift motion (1753 and 176o)
.
Robertson's rule of computing the See also:equation of equal altitudes is as good as any used at the present day
.
He gives also a description of an equal-altitude instrument, having three horizontal wires, probably such as was used at Portsmouth for testing Harrison's timekeeper
.
The mechanical difficulties must have been great in preserving a perpendicular See also:stem and a truly horizontal sweep for the telescope
.
It gave place to the improved sextant and artificial horizon
.
The second edition of Robertson's work in 1764 contains an excellent dissertation oa the rise and progress of modern navigation291
by Dr See also: (It is See also:strange that while reckoning was so rough and imperfect in many respects such a trifle as that is in See also:low latitudes should be noticed.) After speaking of meridional parts, he offers to explain the English method, which was discovered by Edmund Halley, but omits the principles upon which Halley founded his theory, as it was " too embarassing." He gives instructions for allowing for currents and leeway, tables of declination, positions of a few stars, meridional parts, &c . It is worthy of remark that, after giving a form for a log-book, he adds that this had not been previously kept by any one, but he thought it should not be trusted to memory . He only re-quires the knots, fathoms, course, wind and leeway to be marked every two hours . He gives a sketch of Halley's quadrant, but without a clamping screw or tangent screw . To ascertain local time at sea by astronomical observations by the altitude of suitably-situated heavenly bodies was an old, well-known and frequently practised operation, so that a comparison could thus be easily made between such local time and the Greenwich time if known at the same instant . The introduction of timekeepers by which Greenwich time can be carried to any part of the world, and the longitude found with ease, simplicity and certainty is due to the invention of John Harrison . The idea of keeping time at sea by watches was no novelty, but the practical difficulty arose from their very irregular rates owing to changes of temperature and the motion of the ship . See also:Huygens had applied pendulums to the regulation of clocks on shore in 1656, and in 1675 his application of See also:spiral springs as regulators of watches made them available for use at sea . William See also:Derham published a scientific description of various kinds of timekeepers in The Artificial Clock-Maker, in 1700, with a table of equations from Flamsteed to facilitate comparison of mean time with that shown by the sun-dial or apparent time . In 1714 Henry See also:Sully, an Englishman, published a treatise at See also:Vienna, on finding time artificially . He went to See also:France, and spent the See also:rest of his See also:life in trying to make a timekeeper for the discovery of the longitude at sea . In 1716 he presented a watch of his own make to the Academy of Sciences, which was approved; and ten years later he went to See also:Bordeaux to try his marine watches, but died before embarking . See also:Julien le See also:Roy was his See also:scholar, and perfected many of his inventions in watchmaking . Harrison's great invention was the principle of See also:compensation through the unequal contraction of two metals, which he first applied in the invention in 1726 of the compensation (gridiron) pendulum, still in use, and then modified so as to fit it to a watch, devising at the same time a means by which the watch retains its motion while being wound up . With regard to the success of the trial See also:journey (see HARRISON, JOHN) to See also:Jamaica in 1761–1762, it may be noted that by the journal of the House of See also:Commons we find that the error of the watch was ascertained by equal altitudes at Portsmouth and See also:Barbados, the calculations being made by Short; these errors came greatly within the limits of the act . At Jamaica the watch was only in error five seconds (assuming that the longitude previously found by the transit of See also:Mercury could be closely depended on, which as we now know, was not the case, the observations being too few in number, and taken with an untrustworthy instrument) . Short at Portsmouth found the whole unallowed-for error from November 6th, 1761, till April 2nd,1762, to be 1m54°•5=18 geographical miles in the latitude of Portsmouth . During the passage home in the " See also:Merlin " See also:sloop-ofwar the timekeeper was placed in the after part of the ship, because it was the dryest place, and there it received violent shocks which retarded its motion . It lost on the voyage home 1°1 49° =16 geographical miles . One might have supposed that Harrison had now secured the prize; but there were powerful competitors who hoped to gain it by lunars, and a See also:bill was passed through the House in 1763 which left an open See also:chance for a lunarian during four years . A second West Indies trial of the watch took place between November 1763 and March 1764, in a voyage to Barbados, which occupied four months; during which time it is said, in the See also:preamble to act 5 Geo . III . 1765, not to have erred to geographical miles in longitude . We only find in the public records the equal altitudes taken at Portsmouth and at Bridgetown, Barbados . William Harrison assumed an average rate of 1° a-day gaining, and he anticipated that it would go slower by I° for every to increase in temperature . The longitude of Bridgetown was determined by N . See also:Maskelyne and C . See also:Green by nine emersions of Jupiter's fj,c t ,See also:satellite, against five of Bradley's and two at Greenwich Observatory, to be 3b 54m 20B west of Greenwich . In See also:February 1765 the commissioners of longitude expressed an See also:opinion that the trial was satisfactory, but required the principles to be disclosed and other watches made . Half the great reward was paid to Harrison under act of parliament in this year, and he and his son gave full descriptions and drawings, upon See also:oath, to seven persons appointed by the commissioners of longitude.' The other half of the great reward was promised to Harrison when he had made other timekeepers to the See also:satisfaction of the commissioners, and provided he gave up everything to them within six months . The second half was not paid till 1773, after trials had been made with five watches . These trials were partly made at Greenwich by Maskelyne, who, as we shall see, was a great advocate of lunars, and was not ready to admit more than a subsidiary value to the watch . A See also:bitter controversy arose, and Harrison in 1767 published a book in which he charges Maskelyne with exposing his watch to unfair treatment . The See also:feud between the astronomer-royal and the watchmakers continued long after this date . Even after Harrison had received his £20,000, doubts were See also:felt as to the certainty of his achievement, and fresh rewards were offered in 1774 both for timekeepers and for improved lunar tables or other methods . But the tests proposed for timekeepers were very discouraging, and the watchmakers complained that this was due to Maskelyne . A fierce attack on the astronomer's treatment of himself and other watchmakers was made by Thomas Mudge in 1792, in A Narrative of Facts, addressed to the first lord of the Admiralty, and Maskelyne's reply does not convey the conviction that full See also:justice was done to timekeepers . Maskelyne at this date still says that he would prefer an See also:occultation of a See also:bright star by the moon and a number of correspondent observations of transits of the moon compared with those of fixed stars, made by two astronomers at remote places, to any timekeeper . The details of these controversies, and of subsequent improvements in timekeepers, need not detain us here . In England the names of John See also:Arnold and Thomas Earnshaw as watchmakers are prominent, each of whom received, up to 1805, £3000 reward from the commissioners of longitude . It was Arnold who introduced the name chronometer . The French emulated the English efforts for the See also:production of good timekeepers, and favour-able trials were made between 1768 and 1772 with watches by Le Roy and F . See also:Berthoud . The marvellous accuracy with which the modern chronometer is constructed is doubtless greatly stimulated by the annual competition at Greenwich, from which the Admiralty See also:purchase for the British navy . These chronometers are all fitted with secondary compensation balances, and it is therefore unusual in the navy to apply any temperature correction to the rate . The perfection obtainable in compensation may be illustrated by the performance of a chronometer at the Royal Observatory in 1886, which at a mean temperature of 50° F. had a weekly rate of 1.6 secs. losing; and on being further tested at a mean temperature of 92° F., it only changed its weekly rate to 2.9 secs. losing . In the See also:mercantile marine cheaper chronometers without secondary compensation are more commonly used, and temperature corrections applied, calculated from a See also:formula originally proposed by Hartnup, formerly of the See also:Liverpool Observatory . Great success attends this mode of procedure, as illustrated by the following facts . From the discussion of the records of performance of the chronometers of the Pacific See also:Steam Navigation Company during twenty-six voyages from London to See also:Valparaiso and back, by giving equal weight to each of the three chronometers carried by each ship, the mean error of longitude for an average voyage of See also:lot days was less than three minutes of arc . As a single instance, in the s.s . Orellana, on applying temperature rates during a voyage of 63 days, the mean accumulated error of the three chronometers was only 2.3 sec. of time . While chronometers were thus rapidly approaching their present perfection the steady progress of astronomy both by the multiplication and increased accuracy of observations, and by corresponding advances in the theory, had made it possible to construct greatly improved tables . In observations of the moon Greenwich still took the lead; and it was here that Halley's successor Bradley made his two grand discoveries of See also:aberration and See also:nutation which have added so much to the precision of modern astronomy . Kepler's Rudolphine tables of 1627 and Street's tables of 1661, which had held their ground for almost ' The explanations and drawings are at the British Museum; and two of his watches, one of which was used by Captain Cook in the " See also:Resolution," are at Greenwich Observatory . In 1767 Harrison estimates that a watch could be made for £loo, and ultimately for £70 or £80.a century, were rendered obsolete by the observations of Halley and his successor . At length, in 1753, in the second volume of the See also:Commentarii of the Academy of See also:Gottingen, Tobias Mayer printed his new See also:solar and lunar tables, which were to have so great an influence on the history of navigation . Mayer after-wards constructed and submitted to the English government in 1755 improved MS. tables . Bradley found that the moon's place by these tables was generally correct within s', so that the error in a longitude found by lunar would not be much more than half a degree if the necessary observations could be taken accurately at sea . Thus the lunar problem seemed to have at length become a practical one for mariners, and in England it was taken up with great See also:energy by Nevil Maskelyne—" the See also:father," as he has been called, " of lunar observations." In 1761 Maskelyne was sent to St See also:Helena to observe the transit of See also:Venus . On his voyage out and home he used Mayer's printed tables for lunar determinations of the longitude, and from St Helena he wrote a letter to the Royal Society (Phil . Trans., 1762), in which he described his observations made with Hadley's quadrant of 20 in. radius, constructed by John See also:Bird, and the glasses ground by See also:Dollond . He took the observations both ways to avoid errors . The arc and index were of See also:brass, the See also:frame See also:mahogany; the vernier was subdivided to minutes . The telescope was 6 in. long, magnified four times, and inverted . Very few seamen in that day possessed so good an instrument . He considered that ship's time should be ascertained within twelve hours before or after observing the lunar distance, as a good common watch will scarcely vary above a minute in that time . This shows that he must have intended the altitudes to be calculated—which would lead to new errors . He considered that his observations would give the longitude within s a degrees . On the 11th of February he took ten observations; the extremes were a little over one degree apart . On his return to England Maskelyne prepared the British Mariner's Guide (1763), in which he undertakes to furnish complete and easy instructions for finding the longitude at sea or on shore,within a degree, by observing the distance between the moon and sun, or a star, by Hadley's quadrant . How far that promise was fulfilled, and the practicability of the instructions, are points See also:worth See also:consideration, as the book took a prominent place for some years . The errors which he said were inseparable from the dead-reckoning " even in the hands of the ablest and most skilful navigators," amounting at times to 15 degrees, appear to be overestimated . On the other hand, the equations to determine the moon's position at time of observation from Mayer's tables, would, he believed, always determine the longitude within a degree, and generally to half a degree, if applied to careful observations . He recommends the two altitudes and distance being taken simultaneously when practicable . The probable error of observation in a meridian altitude he estimated at one or two minutes, and in a lunar distance at two minutes . He then gave clear rules for finding the moon's position and distance by ten equations, too laborious for seamen to undertake . Admitting the requisite calculations for finding the moon's place to be difficult, he desired to see the moon's longitude and latitude computed for every twelve hours, and hence her distance from the sun and from a proper star on each side of her carefully calculated for every six hours, and published beforehand . In 1765 Maskelyne became astronomer-royal, and was able to give effect to his own See also:suggestion by organizing the publication of the Nautical Almanac . The same act of 1765 which gave Harrison his first £1o,000 gave the commissioners authority and funds for this undertaking . Mayer's tables, with his MS. improvements up to his death in 1762, were bought from his widow for £3000; £300 was granted to the mathematician L . See also:Euler, on whose theory of the moon Mayer's later tables were formed; and the first Nautical Almanac, that for 1767, was published in the previous year, at the cost and under the authority of the commissioners of longitude . In 1696 the French nautical almanac for the following year appeared, an improvement on what had been before issued by private persons, but it did not See also:attempt to give lunar distances.' In the English Nautical Almanac for 1767 we find everything necessary to render it worthy of confidence, and to satisfy every requirement at sea . The great achievement was that of giving the distance from the moon's centre to the sun, when suitable, and to about seven fixed stars, every three hours . The mariner has only to find the apparent time at ship, and clear his own measured lunar distance from the effects of parallax and refraction (for which at the end of the book are given the methods of Lyons and Dunthorne), and then by simple proportions, or proportional logarithms, find the time at Greenwich . The calculations respecting the sun and moon were made from Mayer's last See also:manuscript tables under the inspection of Maskelyne, and were so continued till 1804.2 The calculations respecting the planets are from Halley's tables, and those of Jupiter's satellites from tables made by Wargentin and published by See also:Lalande in 1759 (except those for the fourth satellite) . The See also:original Nautical Almanac contained all the principal points of information which the seaman required, but the great value of such an See also:authentic publication to the whole astronomical world led soon to a considerable increase to its contents . As much of this was unnecessary for the ordinary requirements of navigation, since 1903 it has been issued in two forms, the larger for observatory purposes, the smaller for the class for whom it was originally intended . Various useful rules and tables were appended to early volumes of the Almanac . Thus that for 1771 contains a method and table for determining the latitude by two altitudes and the elapsed time (first published by See also:Cornelius See also:Downes of See also:Amsterdam in 1740) . At the end of the Almanac for 1772 Maskelyne and Whichell gave three special tables for clearing the lunar distance; still their rule is neither short nor easily remembered . An improvement of Dunthorne's solution is also given . In the edition for 1773 a new table for equations of equal altitude was given by W . See also:Wales . In those for 1797 and 1800 tables were added by John Brinkley for rendering the calculations for double altitudes easier . The plan of the Nautical Almanac was soon imitated by other nations . In France the Acad6mie Royale de Marine had all the lunar distances translated from the British Nautical Almanac for 1773 and following years, retaining Greenwich time for the three-hourly distances . The tables were considered excellent, and national See also:pride was satisfied by their having been formed on the plan proposed by Lacaille . They did not imitate the mode given for clearing the lunar distance, considering their own better . Though the Spaniards were leaders in the art of navigation during the 16th and 17th centuries, it was not till November 4, 1791, that their first nautical almanac was printed at Madrid, having been previously calculated at Cadiz for the year 1792 . They acknowledge borrowing from the English and French . The excellent See also:Berlin Aslronomisches Jahrbuch began to appear in 1776, the See also:American See also:Ephemeris in 1849 . These two ephemerides and the French Connaissance See also:des temps are See also:independent and valuable See also:works . A book of Tables Requisite to be Used with the Nautical Ephemeris was published by Maskelyne at the same time as the first Almanac, and ten thousand copies were quickly sold . A second edition, pre-pared by Wales, appeared in 1781, an See also:octavo of 237 pages, in the See also:preface of which it is stated that it contains everything necessary for computing the latitude and longitude by observation . There are in all twenty-three tables, the traverse table and table of meridional parts alone being deficient as compared with modern works of the kind; dead-reckoning Maskelyne did not See also:touch . He gave practical methods for working several problems; that for computing the lunar ' The French nautical almanac or Connaissance des temps appeared under letters patent from the king, dated 24th March 1679—seventeen years before the first issue . The following is a literal translation of its See also:advertisement: " This little book is a collection of See also:holy days and festivals in each month . The rising and setting of the moon when it is visible, and of the sun every day . The aspects of the planets as with respect to each other, the moon and the fixed stars . The lunations and eclipses . The difference of longitude between the meridian of Paris and the principal towns in France . The time of the sun's entrance into the twelve signs of the zodiac . The true place of the planets every fifth day, and of the moon every day of the year, in longitude and latitude . The moon's meridian passage, for finding the time of high water, ' as well as for the use of dials by moonlight.' A table of refraction . The equation of. time [this table is strangely arranged, as though the clock were to be reset on the first of every month, and the explanation speaks of the ' premier See also:mobile '] . The time of See also:twilight at Paris . The sun's right ascension to hours and minutes . The sun's declination at noon each day to seconds . The whole accompanied by necessary instructions." 2 Mayer's tables were printed at London under Maskelyne's superintendence in 1770.especially is an improvement on those by Lyons and Dunthorne, and a rule given for clearing the distance, called Dunthorne's improved method, is remarkably short . Maskelyne's rule for finding the latitudes by two altitudes and the elapsed time is also good .
The third edition of the Tables was issued in 1802
.
The publication of the Requisite Tables met a great want, and the existence of such accurate and conveniently-arranged mathematical tables for the special purposes of nautical calculations led to the more general use of many refinements which had been previously neglected
.
They formed the original of many subsequent and greatly extended collections, of which those by J
.
W
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Norie are the more generally used in modern times in the mercantile marine, and the very accurate and comprehensive tables by James Inman (originally published in 1823) are constantly used in the British navy
.
Until the middle of the 17th century mariners generally employed small collections of Dutch charts, known as " waggoners " from Waghenair, the name of a celebrated Dutch hydrographer in 1584
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In 1671 appeared the English Pilot by John Sellers, who is styled the
Hydrographer Royal." It forms a collection of rude sketches of the coasts of England, the North Sea, France and Spain, with safling directions, and on its appearance the importation of Dutch charts was prohibited
.
Private enterprise, for many years after that, supplied both the British navy and the British mercantile marine with constantly improving charts, especially latterly, under the powerful patronage of the East India Company, whose hydrographer (See also: The expense was then limited to 65o a year . The first See also:official catalogue of Admiralty charts was issued in 183o, the total number being then 962 . After the close of the long devastating war in 1815 both See also:trade and science revived, and several governments besides that of Great Britain saw the necessity of surveying the coasts in various parts of the globe; the greater portion of the work fell to the English hydrographical department, which took under its charge nearly every place where the inhabitants were not able to do it for themselves . Since that time its career of usefulness has steadily developed, and it not merely undertakes the See also:constant improvement of the charts of the whole world, but periodically issues for the use of the seafaring community a vast amount of most accurate and practical nautical information on the various closely allied subjects of navigation, tides, compass See also:adjustment and ocean See also:meteorology . A knowledge of the times and heights of high and low water and the directions of the tidal streams due to those phenomena are in many parts of the world (and especially round our own coasts) of vital importance to navigation . The theory of the tides was first laid down by Newton and See also:Laplace, and in Phil . Trans., 1683, there is an account of Flamsteed's tide table for London See also:Bridge, which gave the times of each high tide on every day in the year . For a long subsequent period empirical tide tables for a few places in England were published by private individuals, but in 1832 the researches of Dr W . See also:Whewell and Sir J . W . Lubbock enabled official tide tables to be issued by the Admiralty . These have steadily advanced in detail and accuracy, being now in many cases based on continuous tidal observations for a whole lunar period of 181 years, and represent the practical epitome of our knowledge of the tides and tidal currents of the whole world . The formulae and tables on which these predictions are based are given in the introduction to each annual volume (see TIDE) . |
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