Page:Popular Science Monthly Volume 19.djvu/71

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fitted at its upper end with peculiar disengaging hooks. The weight was slung to these hooks by means of a wire which passed from a ring slipped over the rod under the weight, up on each side of the cannon-ball to the hooks. The sounding-line was attached to eyes in these hooks, and, as long as the lower end of the rod was not resting on anything, the weight was kept securely in its place, and was available for taking out the sounding-line. As soon, however, as bottom was reached, and the rod came to be supported on its lower end, the hooks at the upper end fell forward, and allowed the wire to disengage itself. The weight was thus released, and, on the line being pulled up, the rod came away through the perforation of the shot, and brought with it specimens of the mud in small quill tubes fitted in a recess in the lower end of the rod. This apparatus has been improved by substituting a tube for the rod, and so arranging the attachment of the weight that it shall continue till the hauling in is begun, whereby its mass and momentum are available for forcing the tube as deep into the ground as possible. Captain Shortland devised another modification of the apparatus in 1868, for the soundings between Bombay and Aden. The essential part was the insertion of two butterfly valves in the lower end, and two conical valves opening upward in the middle of the tube, between which a sample of the bottom water is secured, while a specimen of the mud is brought up in the lower segment of the tube. It was used with general satisfaction during the first year of the cruise of the Challenger. The chief objection to it was founded on the smallness of the samples of bottom which it brought up. This machine, the "Hydra," was replaced after the first year by the "Bailey," an apparatus having a larger tube fitted to bring up more considerable samples of mud.

An apparatus which the author has devised for sounding the Scottish lakes, and found to act well, consists of a straight brass tube an inch in diameter, carrying a shoulder about one foot from the lower end. A cylindrical leaden sinker of suitable weight is slipped over the upper end, and rests on the shoulder. The line is made fast to an eye at the top of the tube, and the part of the tube below the shoulder can be unscrewed, and the mud which it has brought up squeezed out. The tubes bury themselves readily in soft mud and clay, and bring up considerable samples.

It is necessary, in making a sounding in deep water, to load the end of the line with such a weight that in the deepest water that may be reasonably expected the velocity of descent shall not be diminished to an excessive extent by the friction of the increasing length of line in passing through the water. Wire has been largely employed for the line, and has great advantages in this respect over hemp. For example, in water of fifteen hundred fathoms a sinker weighing three hundred-weight is twenty minutes in reaching the bottom, with the best hempen sounding-line; while with wire and a sinker of thirty