Fourier's Theory of Heat. 



603 



determine, by reasoning alone, the exact relation which exists be- 

 tween the duration of the period and the depth to which it is re- 

 quisite to descend, to be beyond the reach of that influence. This 

 relation calculation only can furnish; and that indicates, that the 

 diurnal variations are felt only at a depth nineteen times less than 

 those where the annual variations cease to be observed. 



All the effects of the solar heat upon the earth are modified by 

 the superposition of the atmosphere, and the presence of the waters. 

 The great motions to which these fluids are liable, render the dis- 

 tribution of it more uniform. The air and the waters, besides, exer- 

 cise an action of another kind upon the terrestrial heat ; like trans- 

 parent bodies placed upon the surface of the globe, they augment 

 its temperature. Offering, in fact, a passage sufficiently free to the 

 luminous heat, they present a greater obstacle to the departure of 

 that which the earth afterwards exhales into space. The air and 

 water thus produce nearly the same effect as ordinary glass, when 

 surrounding a body exposed to the sun; or the effect of double 

 sashes upon the temperature of our rooms. We proceed to another 

 cause of the temperature of our globe. 



Numerous observations, now sufficiently ascertained, prove, that, 

 at each point of the earth, the fixed temperatures increase in propor- 

 tion as we descend to the lower depths. But we have seen, that 

 this elevation of the fixed temperature in the direction of the depth, 

 cannot, in any way, be the consequence of the prolonged action of 

 the rays of the sun. The cause which give to the deep strata a 

 fixed temperature, more and more elevated, is, then, an interior 

 source of heat, whether constant or variable, placed below these 

 points of the globe which we have been able to reach. This cause, 

 penetrating to the surface, raises its temperature above that which 

 would be the result of the single action of the sun. But the 

 increase of the temperature, communicated to the superficies by this 

 cause, is almost nothing. This, M. Fourier has demonstrated with 

 mathematical precision ; and, it is a remarkable circumstance, that 

 scarcely had we acquired some certainty as to the existence of a 

 central fire, when the theory of this great geometer furnished us 

 with the means of arriving at the most curious results, as to all the 

 consequences to be drawn from it. 



