Page:Popular Science Monthly Volume 3.djvu/272

260 the weight runs down. Another method of measuring the depth consisted in running down a weight attached to a line, which was cut at the surface as soon as the weight was supposed to have  Fig. 2. The Bull-dog Sounding-Machine. reached bottom, from a sudden change in the rate of running out, and the depth was then calculated by the length of cord left on the reel.

The ordinary system of sounding fails at great depths, and cannot be depended upon for more than 6,000 feet. The weight is not sufficient to carry the line rapidly and vertically to the bottom, and, if a heavier weight be used, the line is in danger of breaking. No impulse is felt when the lead strikes the bottom, and the line goes on running out, and, if stopped, is liable to break. Sometimes the line is carried along by submarine currents, forming loops or bights, and it often continues to run out and coil itself in a tangled mass directly over the lead. These sources of error vitiate very deep soundings, so that the reports that have been made of measurements in the Atlantic of 39,000, 46,000, and 50,000 feet, without reaching bottom, are now regarded as exaggerations. In the last charts of the North Atlantic, on the authority of Rear-Admiral Richards, no soundings are entered beyond 24,000 feet, and very few beyond 18,000 feet.

The ordinary deep-sea lead, which is a prismatic block about two feet in length, and from 80 to 120 pounds in weight, has a simple provision for bringing up material from the bottom, which is called "arming" that is, the lower end, which is slightly cupped, is covered with a thick coating of soft tallow. If it 5 reaches the bottom, mud, shells, gravel, 21125 ooze, or sand, sticks to the tallow, and, when drawn up, affords a sample of the nature of the ground. As the interest in the bottom of the sea increased, there was a more eager curiosity to scrutinize the particles thus procured for chemical and microscopical