72 THE BOTTOM OF THE SEA. 



length of time. Thus tho phiy of the })olar waters 

 makes itself known to the mariner by a lall in the 

 temperature of the sea, and the equatorial currents 

 carry with them into the arctic zones a warm and 

 grateful reminiscence of the sunny climes from which 

 they have descended. 



In general, the density of a current is in inverse 

 proportion to its degree of heat, and accordingly, 

 overlying currents show a decreasing temperature 

 according to their depth. But in the polar seas this 

 is not so. The temperature there increases with the 

 depth, within certain limits, and thus assists in fusing 

 the lower part of the ice. To account for this 

 anomaly, snow and ice are bad conductors of heat. 

 Farmers and gardeners are well acquainted with the 

 fact that a covering of snow keeps the earth warm in 

 winter. The effect of ice in the arctic regions is similar. 

 It has been proved by thermometric observations that, 

 at a surface temperature of 52° below zero (Fahr.), 

 the water under a bed of ice thiity-two feet thick re- 

 mained relatively hot, its temperature not liaving 

 descended below 28° 4' Fahr. 



Thus sea-water is hottest near the surface in the 

 neighbourhood of the equator, while in the arctic 

 regions it is hottest at a certain depth, and coldest in 

 its upper stratum. The temperature of the sea-bottom 

 is, however, uniform over all the world, and differs 



