HOW THE SEA-DEPTHS ARE EXPLORED. 263 



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 ver- 

 tical descent by self-registering machinery. Massey's sounding-ma- 

 chine 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 at- 

 tached, and to the lower the weight at about a yard from the ma- 

 chine. 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 posi- 

 tion 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 the slide (D) is withdrawn.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 mo- 

 tion to the indices, which are so adjusted that the index on the right- 

 hand dial passes through a division for every fathom of vertical de- 

 scent 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 wheel- 

 work. 



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 sur- 

 veyed 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 rep- 

 resent 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 in- 

 cluding 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 



