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Fourier s Theory of Heat. 



it was merged, had been, for a very great number of ages, subjected 

 to the action of the solar rays ? In order to resolve this ques- 

 tion, we ought, evidently, to distinguish the effects produced on 

 the extreme surface, from those which must have occurred at depths 

 more or less considerable. As to the former, nothing is more 

 simple." 



The alternations of the presence and absence of the sun, must have, 

 from the origin of things, occasioned diurnal and annual variations, 

 similar to those which we now observe. Any detail upon this 

 subject, would be superfluous. Every one comprehends, in fact, how 

 the surface, heated by the presence of the sun above the horizon, 

 becomes cooled every night after the setting of that orb. The cause 

 of the annual variations is no less evident. The sun, in our climates, 

 being each day, during the summer, a longer time above the horizon, 

 and darting its rays more directly upon us, a more considerable 

 heating must ensue from this double cause, than that which occurs 

 during the winter, when the sun, notwithstanding its greater 

 proximity to the earth, produces less effect. These phenomena, 

 in their generality, at least, have long been the subject of scientific 

 consideration. 



We shall only observe, that the difference between the heat of 

 the day, and that of the night, and between that of the summer and 

 winter, as to each region, could only be explained by the con- 

 sideration of the influence which the temperature of the planetary 

 spaces exercises upon it, and which no one, before M. Fourier, ever 

 even attempted to estimate. 



The periodical effects, which we have mentioned, are remarked 

 only at the extreme surface, and it is sufficient to penetrate a 

 few feet beneath it to find them very sensibly modified. By virtue 

 of a general law of nature, the strata more immediately below 

 the surface draw from it a portion of the heat communicated by the 

 sun; and the same effect is produced upon the successive strata 

 to a depth which essentially depends upon the time that has elapsed 

 since the period when the heating cause began to operate. 



But the strata heated by imbibing the heat of the superficies, 

 are not liable to the same variations of temperature as this last. 

 To render this fact evident, let us suppose a depth, such that the 



