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NAVIGATION (from Lat. navis, ship, an...
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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
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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
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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
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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
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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
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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.)
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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
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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
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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
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52 3 (51 31) 19 15 (25 54)
See also:Malta 34 0 (35 43) 38 45 (40 31)
See also:Rome
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. 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
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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 |