208 THE DEPTHS OF THE SEA. (CHAP. V. 
South Atlantic at a depth of 46,000 feet;’ and 
Lieutenant Parker, of the U.S. frigate ‘ Congress,’ ran 
out a line 50,000 feet without reaching the bottom.’ 
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 Noy. 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 lb. 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 specialty fitted at con- 
IS oex cit: 2 Loe. cit. 
