XI()DER\ ART OF NAVIGATION.
In the merchant service vessels are taken in and out. of port by a pilot. When the is ready to leave port, the pilot (or captain in the United States Navy) takes charge. Men are stationed at the wheel for the purpose of steer ing and others at the lead to take soundings. Then by means of buoys, beacons, landmarks, ranges, etc., the ship is kept in the channel. When there is any danger that she may touch bottom, sound ings are taken steadily at short intervals. For convenience in entering or leaving port at night, harbor lighthouses and lighted buoys are placed in proper 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 ship is kept in the deep water of the channel. :Many of these ranges are formed by two lighthouses and can therefore he used at night.
When the outer buoys and dangers arc 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 may 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 inueli over one hundred fathoms: (d) ob servations of the heavenly bodies.
Navigation 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 Smi.txos.) 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 chart. it is possihle to obtain the position 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 charts of lunch frequented coasts are generally very reliable.
When far from land, the most accurate nay 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 tae latitude about noon. when the sun bears north or south. The altitude of the sun above the horizon is measured by means of a sextant (q.v.) and the exact time ascertained by means of a chro nometer regulated to Greenwich (in American and British vessels) time. The chronometer 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. disc. = 90° — alt.) : the declination of the sun from the equator can be obtained from the Arantiral Almanac: and the latitude is known with sufficient accuracy from previous observa tions and (lead reckoning. By solving the oblique spherical triangle whose vertices are ai the pole, the zenith, and the mine 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 difference between this and the Greenwich time of the chronometer gives the longitude in time, which is reduced to degrees in the usual manner.