280 PROGRESS OF SCIENCE IN THE CENTURY. 



While the shallow-water areas down to the 100- 

 fathom line are now known with much exactness in 

 many parts of the glohe, there is naturally much less 

 certainty in regard to the deeper parts, though, as 

 Sir John Murray remarks, some 10,000 deep sound- 

 ings were taken in the last decade of the nineteenth 

 century. He estimates that considerably more than 

 half of the sea-floor (103,000,000 square geo- 

 graphical miles in all) lies at a depth exceeding 

 2,000 fathoms, or over two geographical miles. 

 There is a relatively rapid descent along the conti- 

 nental slopes between 100 and 1,000 fathoms, and 

 there are over forty known depressions of more than 

 3,000 fathoms. The greatest known depth is in the 

 S. Pacific, to the east of the Kermadecs and Friendly 

 Islands, 530 feet more than five geographical miles, 

 or 2,000 feet more below the level of the sea than the 

 top of Mount Everest is above it. 



Direct observations with deep-sea thermometers, 

 and indirect inferences from the electric resistance 

 of the telegraph cables lying on the floor of the 

 oceans, show that about 92 per cent, of the entire 

 sea-floor has a temperature less than 40 Fahr. The 

 surface-water cooled at the poles, spreads over the 

 floor towards the equator, carrying with it the oxygen 

 which makes abyssal life possible. Since the light 

 as well as the warmth of the sun does not penetrate 

 below the superficial layers of water, the deep-sea 

 area is dark as well as cold. Therefore there are 

 no plants (apart from some doubtful forms and the 

 resting stages of two or three AlgaB), and this implies 

 that the abundant deep-sea animals depend in the 

 long run on supplies which sink downwards from 

 the populous surface or the crowded shore-areas. 



Especially by Sir John Murray and the Abbe 



