36 THE DEPTHS OF THE SEA. [CHAP. i. 



which is one of the greatest interest in connection 

 with the distribution of marine animals, will he fully 

 discussed in a future chapter. The hroad conclusions 

 to which we have been led by late investigations are, 

 that instead of there being a permanent deep layer of 

 water at 4 C. the average temperature of the bottom 

 of the deep sea in temperate and tropical regions is 

 about 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. Prom cases 

 which are recorded, chiefly by the earlier American 

 sounding expeditions, of the sounding-line having been 

 run out into long loops in soundings where, from the 

 nature of the sea-bed, the bottom water appeared .to 

 be still, it would seem that there are also in some 

 places intermediate currents ; but with reference to 

 their limits and distribution we have as yet no data. 

 That a cold flow from the polar seas passes over the 

 bottom seems to be proved by the fact that in all 

 parts of the world wherever deep temperature sound 

 ings have been taken, from the arctic circle to the 

 equator, the temperature sinks with increasing depth, 

 and is lower at the bottom than the normal tempera 

 ture of the crust of the earth ; an evidence that a 

 constantly renewed supply of cold water is cooling 

 down the surface of the crust, which, being a bad con 

 ductor, does not transmit heat with sufficient rapidity 

 to affect perceptibly the temperature of the cold in 

 draught. It is probable that in winter, in those parts; 

 of the arctic sea which are not directly influenced by j 



