146 FOUNDERS OF OCEANOGRAPHY 



per annum, and Sir John Murray has calculated that at this 

 rate the whole of the land would be transferred to the sea 

 in 6,340,000 years, and the " hydrosphere " would then com- 

 pletely cover the " lithosphere " to a depth of about 1,450 

 fathoms. The whole area of the sea bottom is estimated at 

 nearly 140 million square miles. 



Depths of the Oceans. 



Our knowledge of the main outHnes of the contours of 

 the ocean floor was gained by the " Challenger " expedition 

 half a century ago ; and the many expeditions since, although 

 they have taken thousands of soundings and have filled in 

 many blanks and made known a few deeper holes, have 

 left the picture very much as it was drawn by Sir Wyville 

 Thomson and his colleagues in 1876. The deepest sounding 

 then was 4,475 fathoms ; the deepest known now is 5,348 

 fathoms, over six English statute miles. 



If the floor of the ocean be divided into 1,000-fathom zones 

 of depth (0-1,000; 1,000-2,000; etc.), by far the largest area 

 is that which Hes at depths of from 2,000 to 3,000 fathoms. 

 The smallest area (only about 6 per cent, of the whole) is that 

 at depths over 3,000 fathoms. These " Deeps," as they are 

 called, of over 3,000 fathoms, are relatively small depressions 

 scattered over various parts of the oceans, and it is appro- 

 priate that we should owe most of the numerical statements 

 and maps dealing with such matters to one of the " Chal- 

 lenger " naturaUsts, Sir John Murray, who continued his 

 oceanographic investigations almost to the present day — his 

 last cruise was in the summer of 1910 and his last pubUcation 

 appeared in 1913. He died early in 1914. 



Murray has defined and named 57 " Deeps," the greater 

 number (32) of which are in the Pacific, the deepest 

 of the oceans ; and the largest and one of the deepest 

 of them is the " Tuscarora Deep," a depression running 

 nearly north and south in the North Pacific to the east 

 of Japan. The " Aldrich Deep " in the South Pacific con- 



