Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/276

264 NAVIGATION keeper ; Kelly, SpJierical Trigonometry and Nautical Astronomy, 1796, 4th ed. 1813, clear and simple ; N. Bowditch, Practical Navigator, 1800, passed through many editions, and is still much used in the United States navy ; J. "W. Norie, Epitome of Navigation, 1803, 21st ed. 1878, long a favourite in the mercantile marine from its simplicity, and because navigation can be learned from it without a teacher ; T. Kerigan, The Young Navigator s Guide to Nautical Astronomy, 1821 ; Inman, Epitome of Navigation, 1821,- with an excellent volume of tables largely used in the British navy, 9th ed. 1854 ; E. Kiddle, Navigation and Nautical Astronomy, 3d ed. 1824, 9th ed., by Escott, 1871, still worthy of its high reputation, it does not include great circle sailing ; J. T. Towson, Tables for Reduction of Ev-mcridian Altitudes, 4th ed. 1854, very useful : H. Raper, Practice of Navigation, 1840, 10th ed. 1870, an excellent book, perhaps the best ; H. Evers, Navigation and Great Circle Sailing, 1850, other works on the same subject by Merrifield and Evers (1868), and Evers (1875); R. M. Inskip, Navigation and Nautical Astronomy, 1865, a useful book, without tables ; T. H. Sumner, A Method of finding a, Ship s Position by two Observations and Greenwich Time by Chronometer, this is set forth as a novelty, but was published by Captain R. Owen, R. N. , early in the century, and practised by many officers ; H. W. Jeans, Navigation and Nautical Astronomy, 1858, without tables, a good deal used in the British navy, Jeans revised Inman s tables in 1873; Harbord, Glossary of Navigation, 1863, enlarged ed. 1883, a very excellent book of reference ; "W. C. Bergen, Practice and Theory of Navigation, 1872 ; Sir W. Thomson, Navigation, a Lecture, 1876, well worth reading. PRACTICAL OR MODERN NAVIGATION. The following outline of navigation, as commonly practised in 1883, is sufficient to enable a person to con duct a ship from one port to another, or at least to show in plain simple terms the principle upon which it is done. There will be no tables or logarithms referred to which are not common to all works on navigation, and in all such books there are instructions for their use. Seamanship forms a separate subject, but there are a few points wherein it is inseparable from this ; no man can be a good navi gator unless he possesses sound judgment in seamanship. It is necessary that the ship should be provided with good compasses, in suitable places, an ordinary log and line, as well as any patent log which may be desired, hand leads, deep-sea leads and lines, log-book, work-book, chronometer, tables of logarithms, &c., Nautical Almanac, sextant, artificial horizon, large parallel rulers, a box of mathematical instruments, charts of the ocean, and enlarged plans of the places the ship is likely to visit. Before the ship leaves her anchorage or other security, the helm should be moved hard over each way, to prove that it is clear, and a leadsman stationed on each side of the ship. When clear of the harbour and adjacent head-, lands, the course is set towards her destination or to obtain an offing. At that time a bearing is taken of one known object and the distance estimated, or, two known objects being in a line, a bearing is taken of a third, or two cross bearings ; the result in either case is entered in the log book, with the exact time. This is called the departure (i.e., from the land), and should be secured before rain or fog intercept the view. Thereafter the rough or deck log book should be marked every hour after the log has been hove (the hour being subdivided when necessary), and the estimated course steered, distance through the water, wind, and leeway (if there be any) carefully entered in their proper columns. The courses may be steered and expressed by degrees, if this is desired. The ship s reckoning is in variably made up from noon to noon of each day; all the principal events are entered in the ship s log-book, which is a very important document. That part of a ship s reckoning which is independent of observations of sun, moon, or stars is called dead-reckoning (D.R.), while the position by celestial objects is designated observed (Obs.). The following illustration of a ship s log and day s reckoning will include all the "sailings" which are usually treated under separate heads. The force of the wind and state of weather are expressed according to a scale, from 1 to 12, and by symbols which have been in general use about fifty years. Variation and local deviation of the compass must be here attended to ; the mode of ascertain ing the amount will be explained further on. All problems in this part of navigation are solved by plane trigo nometry, as if working on a plane, except in finding the proportion between difference of longitude and departure (the latter from the meridian in nautical miles). The accompanying example of a ship s log represents the greater portion of a page, which would contain the record of what had occurred during two civil days, and therefore embraces one com plete astronomical day. While the ship remains on the same course the entry should not be repeated ; the open space shows more clearly the amount of distance due to each. The "North Star" is here supposed to have been under way and clear of the harbour at noon, when the departure was taken from the Eddy- stone, the position of which is 50 11 N. and 4 15 W. The courses, are intended to comprise many varieties of circumstances. If a ship be under steam or has a fair wind, these will be but few ; but leeway will always exist according to the strength of the wind, if on the side, though scarcely perceptible by the wake if the ship is going fast. It is supposed that a ship either sailing or steaming even 15 miles an hour, with her broadside to the breeze, will be carried the same distance at a right angle to her course as if she had been stopped during the same period surface of hull, sails, &c., exposed being similar in each case. It is difficult to estimate the drift of a ship during a gale, but it is very seldom overrated. Courses and distances within 50 miles are usually obtained by laying the edge of the parallel ruler on the two positions and run ning it to the compass, which is drawn on the chart ; then tak& the space between the places in the dividers and apply it to the scale of latitude opposite them : the number of miles there indicated will be the distance. Distant places on Mercator s chart, when so treated, will give the course and distance approximately. To Work the Reckoning Arithmetically from the Ship s Log. The true bearing of the Eddystone is reversed and treated as a course. To correct the compass courses, westerly variation and deviation are both applied to the left of the course steered, that is, against the sun, the reverse when they are easterly. In the example the variation is taken as 21 westerly throughout the day s run. The deviation is taken from a table of corrections for a compass to which a magnet had been applied (see COMPASS and MAGNETISM). The first course on the ship s log is W. by S. S. or S. 73 8 W. Apply variation - 21 ; deviation (5 30 ) being easterly is + ; leeway (5 37 ) as the wind is south is also + : result S. 63 15 W. In a similar manner deal with the other courses, marking well the change of sign east or west, and the direction of the wind with regard to the leeway. The direction of the current being true is entered as S. 79 W. After all the courses and distances for tho day have been correctly entered in a traverse table, the correspond ing differences of latitude and departure are taken from the tra verse table of the epitome, and placed in the proper columns. If the angle be less than 45, read difference of latitude and departure at the top of the page ; if greater than 45 it is reversed. As the table is arranged in decimals, 2 5 can be treated as 25 or 250, or conversely, so that large distances may be embraced. This process is called working by inspection, and is sufficiently accurate for the purpose of finding the course and distance made good, or the course and distance, to places within 300 or 400 miles. Traverse Table. Magnetic Course. Tnic Course. Distance. X. S. E. W. N 34 W 1 8 5 S 79 W 5 l" 4-9 WbSS. S 63 W 16 7 3 14-3 sws. S 30 W 14 12-1 7 SMVfW. South. 29-6 29-6 South. S 23 E. 12 11 4-7 SSE. S 53 E. 11-3 6 8 9 WNfN. WHW. West. West. 16-5 ) 16-5 f 33 NW. N G7 W. 21-2 8-3 19-5 NW. N 64 W. 19-5 S-o 17-5 SWV. S 43 W. 17-7 12-9 12-1 17-6 80-7 13-7 108-8 17-6 13-7 G3 l 95-1 As the difference of latitude is south, it must be subtracted from the latitude left. Eddystone, 50 ll DN. 4 15 W. -1 3jl . +2 27 49 7-9-49 7 54" ~6~42