SOUNDINGS are properly the depths of water in rivers, harbours, along shores, and even in the open seas ; but the term is also applied to the nature of the ground at the bottom of the water. If the depth of the water is comparatively small, a man who is stationed for the purpose in the main or wizen chains, on the windward side, throws out a mass of lead, usually in the form of a frustum of a cone, and weighing 8 or 9 lbs., which is attached to one end of a line'bctween 20 and 30 fathoms in length. On this line are fixed, at intervals of two or three fathoms, pieces of leather or cloth of different colours ; and the mark which is next above the surface of the water when the lead strikes the bottom affords an indication of the depth. That which is called the deep-sea lead weighs from 25 to 301bs., and is attached to a line of great length, on which, at intervals, are knots indi • eating the depths. The bottom of the lead is covered with a coating of tallow for the purpose of ascertaining, by the sand, shells, or other matter which may adhere to it, the nature of the ground.
When soundings are to be taken in the survey of a coast, a harbour, or the mouth of a river, the surveying ship and its boats are disposed at convenient distances from each other (suppose from two to five miles) ; their distances with respect to each other and to remarkable objects on the shore being deter mined by the usual trigonometrical observa tions. The boats then row or sail along the directions of the lines joining each other, sounding as they proceed at equal intervals, suppose ten minutes, of time; and thus the outline of the shoal, reef, coast, or river, will be determined, as well as the depth of the water. All the soundings must be afterwards reduced to the depths below the surface of the sea at the level of low water.