36 THE DEPTHS OF THE SEA. [crAP. 1. 
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° C. 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. From cases 
which are recorded, chiefly by the early 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 
