Page:Popular Science Monthly Volume 44.djvu/347

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335
HOW THE SEA IS SOUNDED.

sea sounding apparatus in which there was attached to the lead, upon the principle of the screw propeller, a small piece of clockwork for registering the number of revolutions made by the little screw during its descent; and it having been ascertained by experiment PSM V44 D347 Sounding cylinder with shot attached.jpgFig. 2.—The Sounding Cylinder with Shot attached. in shoal water that the apparatus in descending would cause the propeller to make one revolution for every fathom of perpendicular descent, hands provided with the power of self-registration were attached to the dial, and the instrument was complete. It worked well in moderate depths, but failed in the deep sea on account of the difficulty of getting it down if the line used were large enough to give the requisite strength for hauling up.

Such was the state of the development of the appliances for measuring the depths of the sea in the middle of the present century, when the idea of using a heavy weight attached to a simple hempen cord was proposed. The plan of stretching a line under the strain of a weight at its lower end from the surface to the bottom underlies the method which is now universally employed for sounding the depths of the sea. In shoal water there is cast from the vessel a plummet in the form of an elongated truncated cone attached to a hempen cord which has been previously divided into feet or fathoms. The line is allowed to run out through the hands of a man who detects, by the sense of touch, the instant when the lead reaches the bottom, and reads the depth by noting the division of the line which corresponds with the surface of the water. By filling a small cavity in the base of the lead with tallow, a quantity of the sand or gravel or mud upon which the lead strikes becomes imbedded in the tallow and gives an indication of the character of the bottom soil.

The rough surface of a rope presents an obstacle to its free passage through the water, and therefore as the depths increase it is necessary to employ heavier weights to carry the line swiftly in a straight course to the bottom, and, moreover, stronger rope to bear the increased weight of the sinker. In great depths the size of the rope which is necessary is such as to present considerable surface to the action of submarine currents, which carry the line