208 THE DEPTHS OF THE SEA. [CHAP. v. 



South Atlantic at a depth of 46,000 feet; 1 and 

 Lieutenant Parker, of the U.S. frigate ' Congress,' ran 

 out a line 50,000 feet without reaching the hottorn. 2 

 In these cases, however, the chances of error were 

 too numerous ; and in the last chart of the North 

 Atlantic, published on the authority of Rear-Admiral 

 Richards in Nov. 1870, no soundings are entered 

 beyond 4,000 fathoms, and very few beyond 3,000. 



A great improvement in deep-sea sounding, first 

 introduced in the United States navy, was the use 

 of a heavy weight and a fine line. The weight, a 

 32 or 68 Ib. shot, is rapidly run down from a boat ; 

 and when it is supposed to have reached the bottom, 

 which is usually indicated with tolerable certainty by 

 a sudden change in the rate of running out of the 

 line, the line is cut at the surface, and the depth 

 calculated by the length of line left on the reel. 



As the great problems of physical geography, the 

 strength and direction of currents, and the general 

 conditions of the bottom of the sea began to acquire 

 more general interest, the particles brought up on the 

 * arming ' of the lead from great depths were eagerly 

 sought for and scrutinized ; it thus became important 

 that a greater quantity should be procured, enough 

 at all events for the purposes of chemical and micro- 

 scopical examination. Many instruments have been 

 contrived from time to time for this purpose, and a 

 vast amount of information has been gained by their 

 use. It has now been shown that dredging on a large 

 scale is possible at all depths, but dredging can only 

 be performed under specially favourable circum- 

 stances, and requires a vessel specially fitted at con- 

 1 Loc. cit. 2 Loc. cit. 



