546 LECTURE LVI. 



and in September, 18. Hence, says Mr. Kirwan,* we may understand 

 the reason of the great frequency of colds in spring and in autumn. 



Some philosophers have supposed the earth to become progressively 

 warmer in the course of ages, while others have imagined that its heat 

 is exhausted. Both these opinions appear in general improbable. The 

 greater heat the earth receives by day, the more it throws off, both by 

 day and by night ; so that in the course of a few ages the heat must pro- 

 bably have attained its maximum. Local changes may indeed arise from 

 local circumstances ; thus, the climate of America is said to have become 

 considerably warmer, since a large part of its surface has been cleared from 

 its dense forests by human labour : and to judge from the descriptions of 

 the ancients, it appears that even in Europe the winters were formerly 

 much colder than they are at present. If, however, Dr. Herschel's opinion 

 of the variation of the heat of the sun be confirmed, it will introduce a 

 great uncertainty into all theories upon the subject : since in these calcula- 

 tions 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 

 temperature 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 freez- 

 ing 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 freez- 

 ing than it is absorbed in thawing ; that the heat, thus extricated, being 

 disposed to fly 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 ; arf&vfor this 

 * Ph. Mag. xvi. 212. 



