Fourier's Theory of Heat. 



609 



more evident than those which arise from the secular cooling of 

 the globe. 



The establishment and progress of human societies, and the action 

 of the ordinary powers of nature, may change, more especially in 

 very extensive countries, the state of the surface of the ground, the 

 distribution of the waters, and the great movements of the atmos- 

 phere. Such effects are calculated to produce a very sensible varia- 

 tion in the amount of the mean heat, in the course of a few years. 

 In general, the clearing and cultivation of the lands, the establish- 

 ment of towns, the operations by which a settled course is given to 

 streams and rivers, the drying up of marshes — in a word, all that 

 ensues from the progress of civilization, tends to augment the tem- 

 perature of a country. This would appear to have formerly happened 

 to Germany, which, in the time of Tacitus, was much colder than in 

 our days; and very recently in the United States, the climate of 

 which would seem to have been very evidently softened during the 

 last half century.* These incontestable facts, which appear, at first 

 sight, to contradict the hypothesis of the gradual cooling of the 

 terrestrial globe, clearly prove nothing against it, since they depend 

 on local causes, the amount of which the theory of heat can appre- 

 ciate with sufficient exactness, while that same hypothesis proves, as 

 we have seen, that the influence of the central fire is nearly nothing 

 on the surface. 



We shall now consider a third cause of the terrestrial heat, which 

 consists in the temperature of the planetary spaces. Suppose, for an 

 instant, that the sun, and all the planetary bodies should cease to 

 exist, the region of the heavens occupied by our solar system, would 

 have a certain temperature, which a thermometer placed in any 

 point of it would indicate. Let us point out the principal facts 

 which led M. Fourier to discover the existence of this heat peculiar 



* Mr. Jefferson speaking of Virginia, says, " A change in our climate is taking 

 place very sensibly. Both heats and colds are become much more moderate, 

 within the memory even of the middle aged. Snows are less frequent and less 

 deep. They do not often lie, below the mountains, more than one, two, or three 

 days, and very rarely a week. They are remembered to have been formerly 

 frequent, deep, and of long continuance. The elderly inform me the earth used 

 to be covered with snow about three months in every year. The rivers which then 

 seldom failed to freeze over in the course of the winter, scarcely ever do so now." 

 — T. 



