Fourier's Theorij of Heat. 



601 



heat communicated to the surface cannot reach it till several 

 days have passed. Then clearly the diurnal variations will be no 

 longer felt. The temperature will be there neither so hot as during 

 the day, nor so cold as during the night, but will assume an in- 

 termediate degree, which will directly depend only upon a mean be- 

 tween the heat of several days, and the coldness of several nights 

 consecutively. A thermometer placed at this depth, (that of most of 

 our vaults) will not vary then in the space of twenty- four hours, as it 

 would at the surface, and will remain steady, during a space of time 

 equal to that of a season, constantly denoting a mean temperature, 

 furnished by the aggregate of the days and nights of that season. 



If we descend still lower, we shall arrive at certain strata, where 

 the transmission of the solar heat will not be able to operate, till 

 after a lapse of time, considerable enough to prevent the alter- 

 nations of the seasons being further felt there ; so that we shall 

 then find a fixed temperature, which will be the mean between that 

 of the seasons : that is to say, exactly that which would be obtained 

 by taking the mean value of all the temperatures observed on the 

 surface at each moment, during a great number of years. This 

 fixed temperature of the deep-seated strata, being once established 

 for each point of the earth, at a certain distance from the surface, it 

 cannot fail (by virtue of the law, in consequence of which, a . hot 

 body brought in contact with a cold one, yields a portion of its 

 heat to the latter), ultimately, to spread itself uniformly over every 

 point, to the greatest depths ; so that the final result of the solar 

 influence, after a sufficient lapse of time, must be the establishment 

 of a fixed temperature for every part of the earth, always extending 

 itself in a similar manner, from the line where the periodical vari- 

 ations cease to be felt, to the centre of the earth. 



It is needless to call to mind, that this fixed temperature being 

 the result of the periodical variations of the superficies, and giving 

 precisely, for each place, the mean value of all the temperatures 

 which succeed each other at the surface, for a long course of years, 

 will not change again, being once established, whatever may be the 

 length of time during which the afflux of the solar rays is continued. 

 In the final state which we have spoken of, all the heat which 

 penetrates by the equatorial regions, is exactly compensated by that 



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