FREEZING AKD THAWING. 69 



ductor of heat ; and thus, while a portion of water re- 

 mains unfrozen, the ice has laid up in that a store of 

 heat which partly helps to dissolve it in the spring ; and 

 thus, except in very high latitudes, there is never more 

 ice accumulated in the average of years than can be 

 again dissolved in the summer. 



The advantages of these properties of water in rela- 

 tion to heat are very great, as is sometimes felt by the 

 drifting and grounding of icebergs upon the shores of 

 northern countries. When one of these masses visits 

 Iceland, or the Shetland isles, its absorption of atmos- 

 pherical heat is so great, that it destroys the summer 

 and spreads disease for a considerable distance around ; 

 and were it not for the power that there is in water to 

 resist the downward progress of congelation, lakes, 

 and even the polar seas, would freeze to the bot- 

 tom, and great part of the earth would be desolate. 

 As matters are constituted, however, there does not 

 appear to be any tendency, at least any rapid tendency, 

 in any latitude to become colder ; and though there be 

 changes in the quantity of the polar ice, and also in 

 the snows and glaciers of those alpine regions, where 

 elevation produces the general effect of high latitude, 

 there is no reason for concluding that, taking a range of 

 several years, there is any change of the general 

 quantity. 



The thawing of the polar ice conspires, with the 

 motion which the water receives from the action of the 

 celestial bodies, to keep up a general circulation of the 

 waters. That motion is northward in the great oceans 

 in summer, and southward in them in winter ; and in 

 both seasons it is augmented by the wind, which, 



