ON CLIMATES AND WINDS. ' 699 



onfirmed, it will introduce a great uncertainty into all theories upon the 

 subject: since in these calculations the original heat of the sun has always 

 been supposed unalterable. 



The sea is less heated than the land, partly because a greater quantity of 

 water evaporates from it, and partly because the sun's rays penetrate to a 

 considerable depth, and have less effect on the surface, while the water is 

 also mixed, by the agitation of its Waves and currents, with the colder water 

 below. It is also more slowly cooled than the land, since, when the tem- 

 perature of the superficial particles is depressed, they become heavier, and 

 sink to the bottom. For similar reasons, the sea is colder than the land in 

 hot climates, and by day, and warmer in cold climates, and by night. 

 These circumstances, however, nearly balance each other, so that the mean 

 temperatures of both are equal, that of the sea being only less variable. 

 Although the process of evaporation must cool the sea, yet when the vapours 

 are condensed without reaching the land, their condensation must compensate 

 for this effect by an equal extrication of heat. 



There is another cause which perhaps contributes in some degree, in tem- 

 perate climates, to the production of cold ; that is, the alternation of freezing 

 and thawing. Mr. Prevost observes that congelation takes place much more 

 suddenly than the opposite process of liquefaction; and that of course the 

 same quantity of heat must be more rapidly extricated in freezing than it is 

 absorbed in thawing; that the heat, thus extricated, being disposed to fl}' off in 

 all directions, and little of it being retained by the neighbouring bodies, 

 more heat is lost than is gained by the alternation: so that where ice has once 

 been formed, its production is in this manner redoubled. This circumstance 

 must occur wherever it freezes, that is, on shore, in latitudes above 35°; and 

 it appears that from about 30° to the pole, the land is somewhat colder than 

 the sea, and the more as it is further distant from it; and nearer the equator 

 the land is warmer than the sea: but the process of congelation cannot by 

 any means be the principal cause of the difference, and it is probable that the 

 different capacity of earth and water for heat is materially concerned in it. 



Since the atmosphere is very little heated by the passage of the 

 sun's rays through it, it is naturally colder than the earth's surface; 



