Page:Popular Science Monthly Volume 19.djvu/70

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red, and blue bunting, and knots. The lead is a long, finely-tempered block, generally weighing fourteen pounds, which has a recess at the thick end, and is perforated at the other end for the reception of the line. This instrument is chiefly used while the vessel is in motion. The leadsman swings the lead vigorously, so as to give it momentum enough to carry it well in advance of the ship before it touches the water. It sinks rapidly while the leadsman's position is advancing to the spot where it touched the water. The depth is ascertained by looking at the marks on the line. This method is effective and correct enough for ordinary purposes, in depths of not more than twelve or fifteen fathoms. Accurate soundings may be obtained by reducing the speed of the vessel as much as possible, in depths which do not much exceed thirty or forty fathoms. In ocean-water, where depths of two or three thousand fathoms are met, the vessel must be kept stationary, and heavier weights than are found sufficient for shallow soundings must be employed.

Deep-sea soundings have received much attention during the last thirty years. The first attempt at them appears to have been made by Captain Constantine John Phipps, during his Arctic Expedition in 1773. He sounded a depth of six hundred and eighty-three fathoms with a lead weighing one hundred and fifty pounds, which appears to have sunk about ten feet into the mud. Determinations of the temperature of the sea water and of its density were made at the same time. Captain John Ross employed, during his Arctic voyage of 1818, one of the earliest satisfactory instruments for bringing up a considerable quantity of the bottom mud in deep water, with which he was able to ascertain the temperature at any depth.

A contemporary of Ross, the younger Scoresby, observed that, when in sounding at great depths the ordinary deep-sea line and lead are used, the increasing weight of line, in proportion as more of it is required, renders less certain the determination of the moment when bottom is reached. He has also left the record of the first observation of the effects of the enormous pressure which is acting under the deep waters. The Americans have introduced the method of using fine twine and a heavy weight, both of which may be sacrificed at every sounding, to obviate the inconveniences arising from overweight of rope. The practice of observing the rate at which successive equal lengths of line pass out has been found useful in cases where ordinary observation or feeling does not suffice to indicate when the shot has reached the bottom. Iron wire was first used instead of twine about 1850, by Lieutenant Walsh, of the United States schooner Taney.

When the surveys for telegraphic cables were begun, it became important to ascertain the nature of the ground at the bottom. The apparatus invented by Midshipman John Mercer Brooke, of the United States Navy, in 1854, answered this purpose. It consisted of a cannon-ball with a hole drilled through it. Through this hole passed a straight rod,