SOUNDING to ascertain the depth of the sea has been practised from very early times for purposes of navigation, but it is only since the introduction of submarine telegraphy that extensive efforts have been made to obtain a complete knowledge of the contour of the ocean-bed. As early as the middle of last century a few deep soundings were recorded in various parts of the world: Ellis made one in 1749 of 891 fathoms off the north-west coast of Africa. But these early results must be accepted only with great Caution, for the methods then in use were not such as to ensure accuracy at any depth greater than a few hundred fathoms. Sir John Ross, the arctic explorer, was much in advance of his times as regarded such investigations ; he invented a "deep-sea clamm" for bringing up a portion of the bottom, and on September 1, 1819, in Possession Bay, made a successful sounding at a depth of 1000 fathoms, which is especially memorable because it was clear, from the organisms which came up entangled in the line, that animal life existed at that depth.
The operation of sounding is readily performed in shallow water by letting down a weight attached to a cord, which is marked off into fathoms by worsted tucked under the strands, the tens and hundreds being indicated by different colours. The bottom of the weight usually presents a hollow, which is filled with tallow, so that a portion of the material from the bottom may be brought up and give an indication of its nature. Some-times a valved cavity is used instead of the tallow. It is easy to see that the longer the line let out the greater will be its friction in passing through the water, the more slowly the weight will descend, and the slighter will be the shock transmitted to the upper extremity when it reaches the bottom; indeed, at what are now considered very moderate depths this becomes quite imperceptible: hence in deep-sea sounding the line is carefully watched as it runs out, and the time each 100-fathom mark enters the water is noted down. Owing to the increasing friction these intervals gradually lengthen, but any sudden incre-ment indicates that the bottom has been reached, for it shows that the weight has ceased to act, and that further descent of the line is due merely to its own gravitation. For instance, in one of the " Challenger " soundings, with a line 1 inch in circumference, and with a weight of 4 cwts. attached, the time occupied in descending from 2900 to 3000 fathoms was 2 m. 10 s.; from 3000 to 3100 fathoms 2 m. 13 s., and from 3100 to 3200 fathoms 3 m. 14s., this sudden increase showing that the bottom had been reached in the interval.
Furthermore, the weight required to sink a line in deep water with sufficient rapidity for purposes of accurate observation is so great that it is found impracticable to bring it up again without putting an undue strain upon the rope or seriously prolonging the operation. Hence in 1854 Brooke, an American, devised an ap- paratus by which the weight was de- tached when it reached the bottom and only a small tube containing a sample of the bottom was brought up. This was in fact a modifica- tion of an apparatus which had been devised by Hooke in the 17th century; he made an arrangement in which a light sphere was sunk by a heavy weight, but was liberated on reach- ing the bottom,the depth being then de- duced from the time which elapsed between the sinking of the globe and its reappearance at the surface. Of the various modifications of Brooke's sounding ma- chine, perhaps the most famous is that con- structed by the black- smith of H.M.S. "Hydra," and commonly known as the "Hydra sounding rod." It was used on the cruises of the "Lightning" and "Porcupine" and during the earlier part of the"Challenger"expedition. This apparatus is shown in fig. 1, where AB is the rod, terminating in a tube below so that it may bring up a sample of the bottom; the weights F fit loosely round it and are supported by the wire E which passes over the stud D, where a spring presses against it, the strength of which is so adjusted that it is unable to displace the wire as long as the strain of the weights is upon it, but so soon as this is relieved by their resting on the bottom the wire is thrust off the stud, and when the line is hauled in the weights and wire are left behind.
An improved apparatus has recently been invented by Mr J. Y. Buchanan, and used by him on board the telegraph ships "Dacia" and "Buccaneer," which can be used either in shallow or deep water, and has the advantage of bringing up samples both of the water and of the mud from the bottom. It consists of a hollow cylinder A, fig. 2, at the top and bottom of which are india-rubber valves H, K, opening upwards, so that water passes freely through them during the descent but is retained as soon as a plug of mud occupies the tube B. The weight EE which surrounds the cylinder is supported by a wire F passing over a peculiar hook D, shown separately in figs. 3, 4, 5; when sounding in shallow water it is not necessary to detach the weight, and the wire is therefore placed as in fig. 3; when working at greater depths the wire rests on the other side of the hook, as in fig. 4,the result being that on the bottom being reached it falls into the upper part of the notch, fig. 5, and continues to press the tube into the mud, but when hauling up commences the wire slips out alto-gether and the weight is left at the bottom. A valve L, M, N is sometimes used to retain the sample of the bottom.
At the present time deep-sea sounding is extensively practised for telegraphic purposes, and is almost entirely conducted by means of wire instead of rope, a method introduced by Sir William Thomson. The friction of the wire in passing through the water is of course very much less than that of rope, and hence it runs out and can be hauled in much more rapidly; a smaller sinker may be used, and in very many instances it can be recovered. It is customary in sounding for cables to make very frequent observations (once in from 1 to 50 miles), for it is found that the laying can be accomplished with much less risk of accident if the contour of the ocean-bed be accurately known. The saving of time by the use of wire is very considerable; but the advantage is not so obvious in running out as in hauling in, because a heavier weight is used to increase the rate, this of course involving a loss of iron sinkers. For instance, an apparatus similar to that mentioned above as being used by the "Challenger" took on an average 24 m. 22 s. to sink 2000 fathoms, whilst in a recent sounding by the "Albatross" the weight ran out 2000 fathoms in 20 m. 30 s. and was hauled in in 21 m. 9 s.,a rate which would have been quite unattainable by the aid of rope. The saving in the matter of sinkers is by no means inconsiderable; instead of 3 or 4 cwts., only 50 to 60 lb are used, and Sigsbee has calculated that this difference is sufficient to pay for any extra loss there may be by the breaking of the wire. Captain Magnaghi of the Italian navy and Captains Sigsbee and Belknap of the American Survey have successfully developed the method of sounding with wire, and owing to its use the last-mentioned officer was able to survey the route from San Francisco to Japan, doing all his sounding by hand, which would have been quite impossible had hempen rope been used. When soundings are made for scientific purposes it is customary to ascertain the temperature, both at the bottom and at intermediate depths, by a thermometer of special construction.
For further information, see Sir Wyville Thomson, The Depths of the Sea (London, 1874); Narrative of the Cruise of H.M.S. "Challenger" (London, 1885); Sigsbee, Deep-Sea Sounding arid Dredging (Washington, 1880); Wille, Norwegian North Atlantic Expedition, pt. iv., "Apparatus and how used,' 1876-78; Mill, The Scottish Marine Station (Edinburgh, 1835); and, for an improved apparatus used on board the "Talisman," La Nature, xii. p. 120, 1884; also the annual Reports of the U.S. Fish Commission. (W. E. HO.)
The above article was written by: W. E. Hoyle, M.A., "Challenger" Expedition Office.