DEEP SOUNDINGS. t 
to the sounding-line, retain and are able to bring up a sample 
of the bottom. 
With the aid of steam, dredging has also been successfully 
carried down to 2,435 fathoms, so that the ocean bed may be- 
come in time as well known to us as the bed of the Mersey or 
the Thames. 
Both sounding and dredging at great depths are, however, 
difficult and laborious tasks, which can only be performed under 
very favourable circumstances, and require a vessel specially 
fitted at considerable expense. 
Many of the early deep soundings in the Atlantic, which 
reported the astonishing depths of 46,000 or even 50,000 feet, 
are now known to have been greatly exaggerated. In some 
cases bights of the line seem to be carried along by submarine 
currents, and in others it is found that the line has been 
running out by its own weight only, and coiling itself in a 
tangled mass directly over the lead. These sources of error 
vitiate very deep soundings; and consequently, in the last chart 
of the North Atlantic, published on the authority of Rear- 
Admiral Richards in November 1870, none are entered beyond 
4000 fathoms, and very few beyond 30600. 
“ The general result,” says Professor Wyville Thomson,* “ to 
which we are led by the careful and systematic deep-sea sound- 
ings which have been undertaken of late years is that the depth 
of the sea is not so great as was at one time supposed, and does 
not appear to average more than 2000 fathoms (12,000 feet), 
about equal to the mean height of the elevated table-lands of 
Asia. 
“The thin shell of water which covers so much of the face of 
the earth occupies all the broad general depressions in its crust, 
and it is only limited by the more abrupt prominences which 
project above its surface, as masses of land with their crowning 
plateaux and mountain ranges. The Atlantic Ocean covers 
30,000,000 of square miles, and the Arctic Sea 3,000,000, and 
taken together they almost exactly equal the united areas of 
Europe, Asia, and Africa—the whole of the Old World—and yet 
there seem to be few depressions on its bed to a greater depth 
than 15,000 or 20,000 feet—a little more than the height of 
Mont Blane; and, except in the neighbourhood of the shores, 
* «The Depths of the Sea,” p. 228. 
