The Thomson sounding machine is of two types, deep-sea and coasting. The latter is now used by nearly all large steamers and by many small ones. It consists of an iron-braced wooden frame work o• casing which incloses a steel drum about 18 inches in diameter and three inches thick. The disks forming the sides of the drum project be yond the circumference, forming a broad deep groove for carrying the wire (3-stranded galvan ized wire rope is now generally used). On each side are cranks for winding in, and on one side there is a friction brake and clutch, while on the other there is a dial, allowing the number of fathoms (0 to 200) out, which is operated by gearing from the axle of the drum. The sinker consists of a lead of the usual shape, weighing about 22 pounds, through which is thrust an iron rod, the whole sinker being 46 inches long from bottom of lead to top of rod. The wire rope is made fast to a fathom or two of small soft line, which is secured at the other end to an eye in the upper end of the sinker rod. The manner of ob taining the depth is independent of the length of wire out, and the depth is registered by means of a Thomson chemical tube, a Tanner-Blish tube, or the depth recorder. The Thomson tube is a slender glass tube, about two feet long, closed at one end, and filled with chromate of silver. It is placed in a slightly larger brass tube, which has holes in it to admit the sea water freely and is lashed to the sinker. The machine is installed near the stern or on the ship's rail. To sound. the sinker is lowered over the stern, the line dropped in a fair leader to insure free running, and when all is ready the brake is tripped by a movement of the crank. The sinker drops rap idly to the bottom and the moment it reaches it the line slacks perceptibly and the reel is stopped. The line is then reeled in. Tf the Thomson tube is used, it is removed front the brass receptacle and laid against a special scale. The sea water
has forced itself in the open end to a distance depending upon the pressure (i.e. the depth) ; as far as it reaches the chemical in the tube is discolored, and this point falls abreast the divi sion of the scale which corresponds to the depth of water. Since the measurement is independent of the amount of wire out, the sounding may be taken with the ship going at full speed if the depth is not too great. The Tanner-Blish is similar to the Thomson tube, except that it con tains no chemical. If the tubes are kept care fully dried the distance the water has risen is easily noted; and by redrying the tubes they may be used over and over again. The depth recorder works on a similar principle, and is at tached to the sinker in the same way. The pressure of the water acts against a piston which compresses a spring and carries a sliding index. When the pressure is slacking the piston returns to its initial position under pressure of the spring, but the index remains at the point of the scale to which it is pushed, so that the depth is read off at once.
The Thomson sounding machine for great depths is similar to the small one, but has a special form of brake which adjusts the tension in accordance with Captain Shortland's rule, and has of course a much greater length of line. The Sigsbee machine is much used in the United States Navy. It differs from the Thomson chiefly in having an automatic spring governor to ease the strain on the wire due to the motion of the ship: though there are other points of dissimilarity. It is the invention of Captain C. D. Sigsbee. of the United States Navy, who has done much deep-sea, depth, and current work in the Atlantic and Gulf of :Mexico. See DEEP SEA EXPLORATION; OCEANOGRAPHY.