DEEP SOUNDINGS. 7 



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 Eear- 

 Admiral Eichards in November 1870, none are entered beyond 

 4000 fathoms, and very few beyond 3000. 



" 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 Blanc ; and, except in the neighbourhood of the shores, 

 • " The Depths of the Sea," p. 228. 



