36 THE DEPTHS OF THE! SEA. [chap. i. 



which is one of the greatest interest in connection 

 with the distribution of marine animals, will be fully 

 discussed in a future chapter. The broad conclusions 

 to which we have been led by late investigations are, 

 that instead of there being a permanent deep layer of 

 water at 4° 0. the average temperature of the bottom 

 of the deep sea in temperate and tropical regions is 

 about 0° 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. Erom 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 



