Page:The New International Encyclopædia 1st ed. v. 14.djvu/355

NAVIGATION. 311 NAVIGATION. stationed at the wheel for the purpose of steer- ing ami oUicrs at the lead to talie siiundings. Then by means of buoys, beacons, landmarks, ranges, etc., the sliip is kept in the cluinnel. W hen there is any danger that she may touch l)oltom, sound- ings are taken steadily at short intervals. Kor convenience in entering or leaving |>ort at night, harbor liglitliouses and liglited buoys are placed in proi)er locations. For convenience in navigat- ing many channels, ranges are established. That is, two objects are placed in such a manner that by steering so as to hold them in line the sliip is kept in the deep -water of the cliannel. ilany of the-^c ranges are formed by two lighthouses and can therefore be used at night. hen the outer buoys and dangers are passed and the vessel is in deep water the pilot is dis- charged. The position of the vessel is then carefully ascertained by compass bearings or other means. This is called taking the departure and niay be done at any time before losing sight of the land. The further navigation of the vessel is effected by means of: (a) dead reckoning (q.v.) ; (b) compass bearings, or horizontal angles, if pro- ceeding along the coast with the land in plain sight; (c) the lead (q.v.), so long as the depth is not much over one hundred fathoms; (d) ob- servations of the lieavenly bodies. Xavigation by means of dead reckoning con- sists in determining the position of the ship by means of the record of courses and distances sailed which is kept in the log-book (q.v.). (.See Sailings.) When the course of a ves- sel is along a coast with objects in sight which may be identified and which are marked on the clnirt. it is possible to obtain the ]X)sition of the ship at almost any time during the day, and, by means of observations of lighthouses, fre- quently during the night. When the water is not deep and the coast not clearly visible, an ap- proximate position can be obtained by soundings, but these are not always to be relied upon except within rather wide limits, whereas by compass bearings of objects on shore or by measuring the horizontal angles between three objects the exact position may be determined if the chart is reliable — and cliarts of much frequented coasts are generally very reliable. Micn far from land, the most accurate way of determining the ship's position at any time is by observations of the heavenly bodies. Of these, the most useful is the sun, but observations of the moon, planets, and stars are also taken. The general practice is to determine the longitude by means of the sun in the morning and after- noon, the time when it bears about east or west being the most favorable, and to determine the latitude about noon, when the sun bears north or south. The altitude of the sun above the horizon is measured bv means of a sextant (q.v.) and the exact time ascertained bv means of a chro- nometer regulated to Greenwich (in -American and British vessels) time. The clironometer may not show the exact Greenwich time, but its error and rate of gaining or losing is known, having been carefully determined in port. From the altitude of the sun its zenith distance is ob- tained (zen. dis. = 00° — alt.) : the declination of the sun from the equator can be obtained from the aiilir<il Altiinnnr: and the latitude is known with sufficient accuracy from previous observa- tions and dead reckoning. By solving the oblique k spherical triangle whose vertices are at the pole, the zenith, and the sun-' and of which three sides (as given above) are known, the hour angle of the sun east or west of the meridian is ascer- tained. Correcting this by adding or subtracting the "equation of time' (derived from the nautical almanac) the mean local time is obtained. The dill'erenite between this and the Greenwich time of the chronometer gives the longitude m time, which is reduced to degrees in the usual manner. The simplest method of obtaining latitude, and the one most commonly used, is what is called "latitude by meridian altituih'." This is taken at the time the sun bears north or south as it crosses the meridian. The observer knows the approximate longitude. A short time before the body reaches the meridian, he begins to take altitudes of it (usiuUly the sun is taken). As soon as the altitude stops increasing and begins to decrease he knows that the sun has crossed the meridian and that the maximum altitude ob- tained is the one sought. If h is the altitude thus obtained and d is the declination of the body obtained from the Xnutieal Alnianiic, then the latitude = 90° — (/i -(- rf) , the sign of d being plus if the body is on the same side of the equator as the ship and niinu.'i if on the opposite side. If a meridian altitude cannot be obtained, a spherical triangle must be solved, in which case the time must be as near noon as possible, other- wise a considerable error in latitude may result from a small one in observation or time. The ship's position when ascertained is plotted (i.e. correctly located) on the proper chart and a line drawn connecting it with the preceding po- sition. The continuous line joining the various positions is called the ship's track and shows her latitude and longitude at the difl'erent parts of her voyage. As soon as a position is determined and placed upon the chart the navigator deter- mines the course for the next day, or next few hours as the ease mav be, by finding the course which must be steered to arrive at the point de- sired. This operation is facilitated by the out- line drawing of a compass placed on the chart, the circumference of which is divided into de- grees. By means of parallel rulers the direction of the new course is transferred to this compass and the correct course ascertained. The course thus obtained is either the true course or mag- netic course, according to the character of the chart compass, some of which are laid down v.ith the north point directed toward the true north and some with it directed toward magnetic north. If the log-book of the ship is carefull.v kept and the speeds and courses aceuratelv determined, the navigator may each day determine not only his position, and the course and run since the previous day, but also the ocean currents, because — neglecting errors in obser'ations, etc. — the dif- ference between the course and run by log (or the dead reckoning) and the actual course and run as determined by astronomical observations is due to current. He also frequently checks the talile of compass errors bv bearings of the sun — the correct directions of the sun being com- ]iuted. Bibliography. The following reference works may be consulted by the reader in search of fur- ther information: Walker. Nnvigntimi (Balti- more. 18S8) ; CofTin, Nnvigntion nvii Nautical As- tronomii, revised by Commander C. Belknap, U.