Page:Popular Science Monthly Volume 3.djvu/275

Rh the weight, the tension on the rope (D) is relaxed, the tumblers fall and release the ring, and the weight falls and allows the elastic band to close the scoops and keep them closed upon whatever they may contain; the rope (D) slips through the weight, and the closed scoops are drawn up by the rope (F)."

The attempt has been often made to measure the amount of vertical descent by self-registering machinery. Massey's sounding-machine is the best for this purpose, and operates upon a principle of screw-motion as it falls through the water. As represented in Fig. 3, two thimbles (F F) pass through the two ends of the heavy oval brass shield (A A). To the upper of these the sounding-line is attached, and to the lower the weight at about a yard from the machine. The screw-motion is communicated by a set of four brass vanes or rings (B), which are soldered obliquely to an axis in such a position that, as the machine descends, the axis revolves by the pressure of the water against the vanes. C represents the dial-plate as seen when the slide (D) is withdrawn. The revolving axis communicates its motion to the indices, which are so adjusted that the index on the right-hand dial passes through a division for every fathom of vertical descent whether quick or slow, and makes an entire revolution for 15 fathoms; while the left-hand index passes through a division on the circle for 15 fathoms, and makes an entire revolution during a descent of 225 fathoms. This instrument answers very well for accurate work in moderately deep water; but at extreme depths it has an uncertainty which seems to be shared by all contrivances involving metal wheelwork.

The main theatre of sounding operations has been the Atlantic Ocean, which, from its relation to the leading commercial nations, and for intercontinental telegraphic purposes, has been more carefully surveyed than any other great body of water. Open from pole to pole, participating in all conditions of climate, communicating freely with other seas, and covering 30,000,000 square miles, it is believed to represent general oceanic conditions, and to contain depths nearly, if not quite, as great as the other ocean-basins of the world, although but little is known, it is true, in this respect, of the Indian, Antarctic, and Pacific Seas. The general result of its soundings would indicate that the average depth of the Atlantic bed is not much more than 1 2,000 feet, and there seem to be few depressions deeper than 15,000 or 20,000 feet, a little more than the height of Mont Blanc. Dr. Thomson sums up the general results of the Atlantic soundings as follows: "In the Arctic Sea there is deep water, reaching to 9,000 feet to the west and south-west of Spitzbergen. Extending from the coast of Norway, and including Iceland, the Faroe Islands, Shetland and Orkney, Great Britain and Ireland, and the bed of the North Sea to the coast of France, there is a wide plateau, on which the depth rarely reaches 3,000 feet; but to the west of Iceland and communicating doubtless with the deep