Sounding. From the earliest times navigators have ascertained the depth of the sea in shallow waters by means of a hand-lead. This consists of a hempen rope, marked off into fathoms by worsted of different colours, to which is attached a leaden weight armed with tallow or having a valved cavity to bring up a sample of the deposit at the sea-bottom. In addition to the hand-line steamers are now often furnished with a machine to ascertain the depth while under way. This consists of a winch, wire rope, fair-lead, and a sinker provided with a glass tube, which, on being removed after a sounding, indicates by the action of sea-water on a chemical coating the hydrostatic pressure on the air in the tube and consequently the greatest depth reached by the sinker. One of the latest inventions to ascertain the depth of water while a ship is in motion is the 'submarine sentry.' This is a kind of submarine kite, which is trawled after the ship at a depth of 25 or even 40 fathoms, and as soon as the kite strikes the bottom the fact is indicated in the chart-room. Very many attempts were made to sound the deep sea before satisfactory results were obtained. Magellan during the first voyage round the world attempted to sound the open ocean in the Pacific. Not having reached bottom in 200 fathoms he naively concluded that he had crossed the deepest part of the ocean. Ellis in 1749 and Mulgrave in 1773 failed to sound the deep sea. Sir John Ross was more successful, for in 1818 he sounded in the Arctic seas in 1050 fathoms, bringing up a specimen of the bottom. Sir James Ross during his Antarctic expedition sounded in 2425 fathoms, and on two occasions no bottom was found with 4000 fathoms of line. There was a great uncertainty about these depths owing to there being no sure indication when the weight reached the bottom. Brooke, an officer of the United States navy, in 1854 gave a great impulse to deep-sea sounding by introducing a detaching weight, the sinker being left at the bottom, and only a small tube with a sample of the bottom being hauled up with the line. A modification of this apparatus was used during the Challenger expedition, 3 or 4 cwt. of iron sinkers being left at the bottom in each sounding. A sudden decrease in the rate at which the rope was running out showed when the sinkers had struck the bottom. The sounding line was inch in circumference, and in addition to the sinkers and sounding tube there were attached to it several thermometers, a water bottle, piezometers, and other instruments. Deep-sea sounding for telegraphic purposes is now carried on by means of wire rope which was introduced by Sir William Thomson (Lord Kelvin). The friction of the wire in passing through the water is much less than that of the hemp rope. It runs out and can be hauled in much more rapidly; a smaller sinker can be used, and this often can be pulled up along with the wire. When only the depth is required a fine twine with a weight is now used in sounding in deep water, the whole being cut adrift when the depth is ascertained. The time employed in hauling in the line is thus saved, which well repays the loss of twine and weight. The deepest soundings and results are noted at SEA, PACIFIC, &c.
See Narrative of the Cruise of H.M.S. Challenger, vol. i.; Deep-sea Sounding and Dredging, by Sigsbee; and Challenger Report on Deep-sea Deposits, by Murray and Renard.