SOUNDING. DEEP SEA. Until within a few years past, the term deep sounding was understood to be that in which a ship sounded to ascertain her position, and where the depth exceeded that which could be obtained with the lead Oirown by the band, or hand lead; but the necessities of telegraphic communication across the ocean, by means of cables containing insulated wires, have caused the ocean to be measured at depths which were never before considered necessary, or even practicable.
The act of obtaining. a deep-sea sounding may be said to consist of two parts-1. To get the sinkers to the bOttora as quickly as possible with the line straight up and down ; and 2. To bring a portion of the soil of the bottom, as a proof, to the surface; this necessitates the-use of a small but strong line, with heavy sinkers and a detaching appa ratus for freeing the sinkers when they reach the bottom, as from the smallness of the line and the great friction of all passing through the water, the strain of bringing the sinkers up would be-too great for its strength. It may be stated that there is no diffi culty whatever in obtaining a sounding, and regaining the sinker with bottom specimen, up to a depth of 1000 to 1200 fathoms (11 nt.), by means of a heavy lead fitted with a valved tube (fig. 1); but when the depth exceeds 2,000 fathoms the difficulties in obtain ing a correct sounding increase in a compound ratio with the depth.
The first detaching apparatus (fig. 2) was devised by Mr. Brooke, a midshipman of the United States navy: it is extremely simple and efficient. It consists of a rod with a movable hook at the upper end, and a tube at the lower end. The sinker is a perforated shot, through which the tube passes, and by means of a ring below the shot the weight is suspended to the hook by wire, the hook being kept up by the sounding-line; the tube is filled with cut quills. When the weight touches the ground, the line is slackened, the hook falls, and the suspending wire beingfreed, the shot slides off, while the quills being thrust into the soil, secure a small portion, which is brought up with the rod.
Many different kinds of detaching ap paratus have been invented since. but that made use of on board H.M.S. Challenger, in her deep-sea exploration voyage, is but a modification of the original Brooke's machine. The Hydra machine (fig. 3) consists of a tube of iron, 21 in. in diame ter and 41 ft. in length. a; the lower 12 in., b, is KT:irate from, but screws to the upper part at c; it is fitted with a butter fly valve at the lower end, to retain the bottom specimen. At the upper end of the tube is a piston-rod d, which moves freely in the tube. To the upper part of this rod is fixed a steel spring, bent in a bow e; a slit in the spring is adapted to the hook! which protrudes beyond the spring when the lat ter is forced back. The sinkers (fig. 4) are cast-iron disks of half a cwt. each, the hole through the center, a, being sufficiently large for the sounding-tube to pass through.
They are made to fit each other by means of small conical protuberances on the one side, and corresponding hollows on the other, b, so that when placed one on another, the groove c in the one weight corresponds. to that on the other. The upper and lower, sinkers differ a little in form.
When a sounding is to be taken the machine is prepared as in Fig. 5, a wooden stand being used for the purpose. The sinkers a, a, are piled to the required weight, say 4 cwt.; the tube is then passed through them, and au iron ring (with a bight of iron wire attached) bb, is passed 'on the lower end of the tube, and the wire led along the contin uous grooves on each side of the Sinkers c, and the bight passed over the hook d, the spring being pressed back. When the weight of the sinkers rests on the ring, and is supported by the wire, the weight keeps the spring pressed iu; but as soon as the sinkers touch the ground and the weight is relieved from the wire, the spring throws it off the hook, and tile tube is drawn clear through the sinkers.
When the tube, with sinkers complete is ready, it is carefully hoisted over the side, lowered gently into the sea, and eased down one or two hundred fathoms before being let go. It is then let go, and the passing of each 100-fathoms mark is timed and recorded in a printed form made to contain all the particulars of the sounding.
Sir William Thomson, F.R.S., has invented a mode of deep-sea sounding by using piano wire instead of hempen lines, which promises to obviate much of the present dia. culty in deep-sea sounding.
Many very deep soundings are on record, but the two deepest well-authenticated aoundings are among those by H.M.S. Challenger. The first was about 80 m. to the northward of the Virgin islands, the depth being 3,875 fathoms, or nearly 41 miles. Unfortunately, not thinking that so near the islands so great a depth would be found, only 3 cwts. of sinkers were used (the usual quantity for such extreme depths being 4 cwts.); this weight, with a one-inch line, took an hour and twelve minutes to reach the bottom. As the ascertainment of the sinkers reaching ,the bottom depends on the time intervals, it may be stated that the line let free to run with this weight would take about 43 seconds running out the first 100 fathoms, and the time increases as nearly as possible three seconds for each successive 100 fathoms; so that when the interval is beyond this rate, the sinker has reached the bottom. On this occasion the last 50 fathoms ran out at the rate of 2 minutes 36 seconds per 100 fathoms. The other was to the n. of New Guinea, where the depth was 4,475 fathoms, or more than 5 miles.
An idea of the average depth of the n. Atlantic ocean may be had from the fact that of 108 soundings obtained by the Challenger, 48 were between 1000 and 2,000 fathoms, 56 between 2,000 and 3,000, while only the other 4 exceeded 3,000. See DREG'GE; DREDGING.