These sources of error vitiate the results of very deep soundings. Thus Lieutenant Walsh, of the U.S. schooner Taney, reported 34,000 feet without touching bottom; and the U.S. brig Dolphin used a line 39,000 feet long without reaching bottom. An English ship reported 46,000 feet in the South Atlantic and the U.S. ship Congress 50,000 feet without touching bottom. These are, however, known to be errors, so that no soundings are entered on charts over 4,000 feet, and few over 3,000. The U.S. Navy introduced the first great improvement in deep soundings. This consisted in using a heavy weight and a small line. The weight, a 32 or 68-pound shot, was rapidly run down, and when it touched bottom, which was shown by the sudden change in the rapidity with which the line was run out, the line was cut and the depth estimated from the length of cord remaining on the reel. This, however, cost the loss of the shot and the line for each sounding.
One of the first attempts at deep sea dredging was made in 1818, by Sir John Ross, in command of the English navy vessel Isabella, on a voyage for the exploration of Baffin's Bay with a machine of his own invention, which he called a "deep sea clamm." It consisted of a pair of forceps, kept apart by a bolt, and so contrived that when the bolt struck the ground a heavy iron weight slipped down a spindle and closed the forceps, which retained a portion of the mud, sand, or small stones, from the bottom. With this instrument he sounded in 1,050 fathoms, and brought up six pounds of very soft mud, using a whale line, made of the best hemp, and measuring 2 1/2 inches in circumference.
The cup lead is another invention. With this there is a pointed cup at the bottom of the lead, fastened to it with a rod upon which a circular plate of leather plays, serving as a cover to the cup. As it strikes the bottom, the cup is driven in the mud, and on hauling up the cover is pressed into the cup by the water, and brings up the mud it contains. The objection to this is that it is too crude; in its passage up, the water washes away the mud, so that only on an average of once in three times does the cup come up with anything in it; and deep sea soundings take too much time, and are too valuable, to admit so large an average of loss.
About 1854 Mr. J. M. Brooke, of the U.S. Navy, who was at the time associated with Prof. Maury, so well known for his labor in gathering and diffusing a knowledge of the currents of the ocean, invented a deep sea sounding apparatus, which is known by his name. It is still in use, and all the more recent contrivances have been, to a great extent, only modifications and improvements upon the original idea, that of detatching the weight. The instrument is very simple. A 64-pound shot is cast with a hole in it. An iron rod, with a cavity in its end, fits loosely in the hole in the shot. Two movable arms at the top of the rod are furnished with eyes holding ends of a sling in which the ball hangs. The cavity at the end of the rod is furnished with tallow, and the apparatus is let down. On reaching the bottom, the rod is forced into the mud, the cavity becomes filled with it, and there being no more tension, on the rope holding up the movable arms, they fall, disengage the ends of the sling, and allow the ball to slide down the rod. The rod is then withdrawn, carrying up the portion of the bottom secured in the cavity at its foot, and leaving the ball on the bottom. This apparatus costs a ball each time it is used, and brings up but a small portion of the bottom, which is also apt to be diminished on its way to the top, by the water it passes through.
STRIKING THE SEA BOTTOM. BROOK'S DEEP SEA SOUNDING APPARATUS.
Commander Dayman, of the English Navy, in 1857 invented an improvement upon Mr. Brooke's original invention. He used iron wire braces to support the sinker, as these detach more easily than slings of rope. The shot he replaced by a cylinder of lead, as offering less surface to the water in its descent, and he fitted the cavity in the bottom of the rod with a valve opening inward. Commander Dayman used the apparatus, with these modifications, in the important series of soundings he made in the North Atlantic, while engaged in surveying the plateau for the ocean telegraphic cable, and reports that it worked well.
