CHAPTER V.

DEEP-SEA SOUNDING.

The ordinary Sounding-lead for moderate Depths. —Liable to Error when employed in Deep Water. - Early Deep Soundings unreliable. Improved Methods of Sounding.-The Cup-lead.Brooke's Sounding Instrument.-The Bull-dog'; Fitzgerald's; the 'Hydra.'-Sounding from the Porcupine.'-The Contour of the Bed of the North Atlantic.

IN all deep-sea investigations it is of course of the first importance to have a means of determining the depth to the last degree of accuracy, and this is not so easy a matter as might be at first supposed. Depth is almost invariably ascertained by some modification of the process of sounding. A weight is attached to the end of a line graduated by attached slips of different coloured buntine (the woollen material of which flags are made, in which the colours are particularly bright and fast) into fathoms, tens of fathoms, and hundreds of fathoms; or, for deep-sea work, with white buntine at every 50, black leather at every 100, and red buntine at every 1,000 fathoms. The weight is run down as rapidly as possible, and the number of fathoms out when the lead touches the bottom gives a more or less close approximation to the depth.

The ordinary deep-sea lead is a prismatic leaden block about two feet in length and 80 to 120 lbs. in weight, narrowing somewhat towards the upper end, where it is furnished with a stout iron ring. Before heaving, the lead is armed,' that is to say the lower end, which is slightly cupped, is covered with a thick coating of soft tallow. If the lead reach the bottom it brings up evidence of its having done so in a sample sticking to the tallow. Usually there is enough to indicate roughly the nature of the ground, and it is on the evidence of samples thus brought up on the arming' of the lead that our charts note 'mud,' 'shells,' 'gravel,' 'ooze,' or 'sand,' or a combination of these, as the kind of bottom at the particular sounding; thus we have 2000s, mud, shells, and sand at 2,000 fathoms ; 2,050 oz. st., Ooze and stones at 2,050 fathoms; m. s. sh. sc., mud, sand, shells, and scoriæ at 2,200 fathoms, and so on.

2,200

3,200'

m. sh.

When no bottom is found, that is to say, when there is no arrest to the running out of the line and nothing on the 'arming' of the lead, the sounding is entered on the chart thus, 200 no bottom at 3,200 fathoms. Such soundings are not to be depended upon in deep water, but they are usually quite reliable for moderate depths, so far as they go. They give us no help in the exploration of the bottom of the sea, but they are of great practical value, and indeed they give all the information which is directly required for the purposes of navigation; for if there be 'no bottom' at 200 fathoms, there is probably no dangerous shoal in the immediate neighbourhood.

Soundings are usually taken from the vessel, and while there is some way on. Where great accuracy

is required, as in coast-surveying, it is necessary to sound from a boat, which can be kept in position by the oars and reference to some fixed objects on shore.

This ordinary system of sounding answers perfectly well for comparatively shallow water, but it breaks down for depths much over 1,000 fathoms. The weight is not sufficient to carry the line rapidly and vertically to the bottom; and if a heavier weight be used, ordinary sounding line is unable to draw up its own weight along with that of the lead from great depths, and gives way. No impulse is felt when the lead reaches the bottom, and the line goes on running out, and if any attempt be made to stop it it breaks. In some cases bights of the line seem to be carried along by submarine currents, and in others it is found that the line has been running out by its own weight only, and coiling itself in a tangled mass directly over the lead. All these sources of error vitiate very deep soundings. In many of the older observations made by officers of our own navy and of that of the United States, the depth returned for many points in the Atlantic we now know to have been greatly exaggerated; thus Lieutenant Walsh, of the U.S. schooner Taney,' reported a cast. with the deep-sea lead at 34,000 feet without bottom; Lieutenant Berryman, of the U.S. brig 'Dolphin,' attempted unsuccessfully to sound midocean with a line 39,000 feet long; 2 Captain Denham, of H.M.S. 'Herald,' reported bottom in the

1

1 Maury's Sailing Directions, 5th edition, p. 165, and 6th edition (1854), p. 213.

2 Maury, Physical Geography of the Sea. p. 309.

Eleventh edition,

1

and

South Atlantic at a depth of 46,000 feet ; Lieutenant Parker, of the U.S. frigate Congress,' ran out a line 50,000 feet without reaching the bottom.2 In these cases, however, the chances of error were too numerous; and in the last chart of the North Atlantic, published on the authority of Rear-Admiral Richards in Nov. 1870, no soundings are entered beyond 4,000 fathoms, and very few beyond 3,000.

A great improvement in deep-sea sounding, first introduced in the United States navy, was the use of a heavy weight and a fine line. The weight, a 32 or 68 lb. shot, is rapidly run down from a boat; and when it is supposed to have reached the bottom, which is usually indicated with tolerable certainty by a sudden change in the rate of running out of the line, the line is cut at the surface, and the depth calculated by the length of line left on the reel.

As the great problems of physical geography, the strength and direction of currents, and the general conditions of the bottom of the sea began to acquire more general interest, the particles brought up on the arming' of the lead from great depths were eagerly sought for and scrutinized; it thus became important that a greater quantity should be procured, enough at all events for the purposes of chemical and microscopical examination. Many instruments have been contrived from time to time for this purpose, and a vast amount of information has been gained by their use. It has now been shown that dredging on a large scale is possible at all depths, but dredging can only be performed under specially favourable circumstances, and requires a vessel specially fitted at con

1 Loc. cit.

2 Loc. cit.