INTRODUCTORY. 13 



abound at a depth of 2,000 fathoms, do bear that 

 extreme pressure, and that they do not seem to be 

 affected by it in any way." 



Secondly, as to temperature. It was generally 

 understood, until about twenty-five years ago, that 

 whatever the variation of surface temperature might 

 be, there was a certain depth at which the tempera- 

 ture was permanent at 4 C., which is the temperature 

 of the greatest density of fresh water. On this point 

 it is unnecessary to recapitulate the details of a long 

 chapter, but to accept the broad conclusions at which 

 Professor Thomson arrived. That, " instead of there 

 being a permanent deep layer of water at 4 C., tbe 

 average temperature of the bottom of the deep sea, 

 in temperate and tropical regions, is about o C., the 

 freezing point of fresh water ; and that there is a 

 general surface movement of warm water, produced 

 probably by a combination of various causes, from 

 the equatorial regions towards the poles, and a slow 

 under-current, or rather indraught of cold water from 

 the poles towards the equator." " The temperature 

 of the sea apparently never sinks, at any depth, 

 below 3'5 C. (3^ Cent, below the freezing point 

 of fresh water), a degree of cold which, singularly 

 enough, is not inconsistent with abundant and vigor- 

 ous animal life, so that in the ocean, except perhaps 

 within the eternal ice-barrier of the antarctic pole, 

 life seems nowhere to be limited by cold." 



