Sounding

SOUNDING. The determination of the depth of the sea has been practised from very early times for purposes of naviga tion, but it is only since the introduction of submarine that extensive efforts have been made to obtain a complete knowl edge of the contour of the ocean-bed (see OCEAN ) .

For depths over 20 fathoms sounding machines are often em ployed and wire has entirely superseded hemp gear. Its smooth surface and minute section, reducing friction to a minimum, give a rate of descent of about ion fathoms per minute. Reeling in may be accomplished at nearly the same rate. Such soundings can be obtained in any weather short of a fresh gale. A sounding of 1,000 fathoms may be obtained in 25 minutes and 3,00o fathoms in 75 minutes. But beyond that depth, great caution is required, the time occupied is increased, and reeling in must be done very deliberately. A sounding of 5,269 fathoms was obtained near the island of Guam by the U.S. cable-surveying ship "Nero." Soundings at such depths may occupy a period of time lasting as long as five or six hours.

Lucas Machine.

Among the sounding machines in general use the Lucas carries nearly 6,000 fathoms of 20-gauge wire, and is fitted with two brakes—one a screw brake for holding the reel when required, the other an automatic brake for stopping the reel when the weights strike the bottom. A guider for winding the wire uniformly is also attached. After leaving the reel the wire passes over a registering wheel, the dial of which indicates the length of wire run out. Smaller machines are used in boats.

Sounding Rods and Sinkers.

Under i,000 fathoms a lead of 3o to 4o lb. weight can be recovered, and no detaching rod is necessary. For greater depths the "Baillie rod" is the best ap paratus. It is a tube about 2 ft. long, having a rounded coni cal top, through which passes a movable steel rod. The lower part of the tube screws on and off, and contains a double flap valve to retain the bottom specimen. The sinkers, each 25 lb. in weight, conical in form, and pierced with a cylindrical hole through which the Baillie rod passes loosely, are slung by wire or cod line secured to a flat ring and passing over a projection on a movable rod to which the sounding wire is attached. The weight of the

apparatus being taken by the sounding wire, the sinkers remain suspended ; but on striking the bottom, the wire slackens, and the weight of the sinkers drags the movable rod down till the projec tion bears against the curved surface at the top of the rod. The wire sling is forced clear of the projection and the sinkers are slipped. In descending, the valves opening upwards, allow the water to pass through freely; but on drawing up they are closed thus retaining the material with which the tube is filled. For depths under 2,000 fathoms two sinkers are sufficient. In deeper water a third cylindrical weight of 20 lb. should be put between them. It is important to interpose a hemp line, some 20 fathoms long, between the end of the wire (into which a thimble is seized) and the lead or rod. This prevents the wire from kinking on the lead striking the bottom.

Method of Sounding.—The machine is placed on a projecting platform on the forecastle. As the wire runs out the brake must be gradually screwed up, so as to increase the power in propor tion to the amount of wire out. The regulating screw is marked for each Soo fathoms. In fairly smooth water the brake will at once act when the weight strikes the bottom and the reel stops. Under 3,00o fathoms one spring is sufficient, beyond that depth two springs are required.

Handling the Ship.—Sounding from forward is preferable as it enables the ship to be handled with greater ease; in heavy weather when impossible to work on the forecastle, soundings may be obtained from a machine mounted over the stern. The ship's head must be kept in a direction which is the resultant of the direction and force of the wind and current ; and this is arrived at by altering the course while sounding, point by point, until the wire can be kept up and down by moving the engines slowly ahead as necessary.