Several types of sounding machines are in use, but whatever the pattern, each contains a reel wound with thousands of fathoms of piano wire. In the operation of sounding, a detach able shot weighing from 30 to 60 pounds is employed to draw the wire rapidly and steadily from its reel. It is secured to the sounding rod at the end of the line in such a manner as to become detached when the bottom is reached. Several instruments are sent down at each operation. The sounding rod is provided with a device for bringing up a specimen of the bottom mud. A few feet above it are attached the deep-sea thermometer and water bottle. The latter secures its sample of water at the bottom only. All deep-sea instruments are necessarily self-acting at the bottom and thermometers are so constructed that they will not be effected by higher temperatures on being brought toward the surface. While the sound ing instruments are carried down by the heavy shot, the wire is reeled in by steam power. It takes about one hour to make a sounding three miles deep and reel in the line with its attached instruments.
Dredging.— For the collecting of animal life from the bottom, the dredge is employed. This may be described as a bag-shaped net about 20 feet long, fitted with an iron-framed mouth from 8 to 12 feet wide and 3 or 4 feet high It is towed by a wire cable about one third of an inch in diameter, its length depend ing upon the region to be investigated. The cable is operated by a powerful engine on the main deck, and the cable itself is coiled upon a reel sufficiently large to contain the whole of it. The dredge sinks by its own weight, which is sufficient to carry down the dredge line with it, the only weights ordinarily used being small ones to carry the end of the net down below its iron-framed mouth and prevent its fouling. When dragging along the bottom, it usually fills rapidly. Very often the hauls of animal forms are large, but the dredge is more often heavily loaded with oozes of various kinds in which animal forms are imbedded. The dredge rope is passed over the side of the ship from the end of a boom, the top of which is guyed to the mast with a spring to relieve strain. The spring or accumulator is graduated and often shows the dredge to be pulling thousands of pounds. The net is sometimes too heavily loaded to be lifted, and may be torn from its frame. Much of the deep-sea mud or ooze washes away during the progress of the dredge toward the surface and under ordinary condi tions its load is no greater than can be lifted from the water and swung on board with safety.
Dredges are of several forms, the common est being that of the beam trawl employed by fishing vessels in shoal water, but with a shorter beam. For convenience we shall refer
to all appliances used for collecting at the bot tom as dredges. For the exploration of in termediate depths, several types of nets are employed which can be opened and closed by devices operated by metal messengers sent down the dredge line. Various devices are used for collecting forms of life in deep water, such as the otter net and other appliances well known to fishermen. The surface and intermediate depths of the sea bear forms of life for the capture of which fine-meshed tow nets are used.
Before using the dredge or beam trawl, the depth is ascertained by sounding. In dragging the dredge upon the bottom it is necessary to let out considerably more line than the actual depth, and the dragging of the dredge along the bottom can often be felt by merely placing the hand upon the dredging cable. The deepest dredge haul ever made (that taken by the Albatross off the Tonga Islands in the South Pacific) was 4,173 fathoms, and the time re quired from the moment the dredge was put overboard until brought to the surface was about 10 hours. On very rough bottom, where dredging nets may be torn from their frames, the °tangle° is often used. This apparatus consists of bunches of shredded rope attached to iron bars, and is effective in obtaining star fishes, sea urchins, crinoids and other creatures which have their surfaces sufficiently rough to become entangled in the loose strands of rope. Fish traps of special design have been used successfully at great depths, and the Albatross has succeeded in taking fishes at a depth of a thousand fathoms with ordinary gill nets.
Deep-Sea Life.—As plant life does not exist in the sea at depths below the influence of sunlight, we find at the bottom and at inter mediate depths, forms of animal life only. These consist of fishes, mollusks, crustaceans, medusas, echinoderms and representatives of practically all of the classes of marine life to be found living in shallow waters under the influence of sunlight, but of very different genera and species. When deep-sea investiga tions began, it was hoped that archaic forms would be discovered; but this hope has not been realized except in slight degree. Animal forms are sometimes brought up by the dredge in great abundance. It is not an uncommon thing to obtain several barrels of crustaceans, starfishes, mollusks or echinoderms, at a single haul. The dredge of the Albatross once brought up 800• fishes from a depth of 1,770 fathoms. The fact that the Albatro.ss obtained living sponges at 4,178 fathoms indicates that there is probably no depth too great for the existence of animal life.