NAVIGATION

(Lat. nav is, a ship), is that branch of science by which the mariner is taught to conduct his ship from one part or place to another. To understand the principles of navigation, and their practical application, it is nece ary that the mariner should be acquainted with the form and magnitude of the earth, the relative situations of the lines conceived to be drawn on its surface, and have charts of the coasts and maps of the harbours which he may have occasion to visit. He must also understand the use of the instruments by which the direction in which a ship is steered and the distance which she sails are ascertained; and be able to deduce from the data supplied by such instruments the situation of his ship at any time, and to find the direction and distance of any place to which it may be required that the ship should be taken. A curve pa ing through any two places on the earth, and cutting every intervening meridian at the same angle, is called a rhumb line; the angle which such a curve makes with each meridian is called the course between any two places through which the curve pa es; and the arc of that curve intercepted between any two places is called their nautical distance. This distance is more than that measured on the arc of a great circle pa ing through the two places, unle both places are on the same meridian,orboth on the equator, when the rhumb line and great circle coincide. The difference of latitude between any two places is an arc of a meridian intercepted between the parallels of latitude on which the places are situated; and the difference of longitude is the arc of the equator, or the angle at the pole included between the meridians of the places. Hence, when the latitudes or the longitudes of two places are of the same denomination with respect to north or south, east or west, the difference is found by subtracting the le from the greater; but when of different denominations, what is called their difference is found by taking their sum. See Latitude, Longitude. When 816a ship has sailed on a rhumb line from one merid- [s. 830]