604 



Fourier's Theory of Heat. 



Perhaps, at first sight, it will appear surprising, that, without 

 knowing either the nature of the focus of the internal heat, its 

 intensity, or the depth at which it is situate, we should be able to 

 determine any thing as to the relative influence which it is capable 

 of exercising upon the surface. But this influence does not depend, 

 directly, upon any of the circumstances which we have related ; and 

 in order to calculate it very closely, it is sufficient to have, 1st, the 

 exact measure of the elevation of the temperature in the strata si- 

 tuated immediately below the soil ; and, 2ndly, to know the degree 

 of facility with which the heat can penetrate each of the substances 

 which compose them. It does not, in fact, require much reflection 

 to understand, that the central fire, whatever it may be, and what- 

 ever its position, not being able to exercise any influence upon the 

 surface of the earth, but by the intervention of the most superficial 

 strata, the effect which it will produce, will have an immediate and 

 necessary relation with its mode of action on the latter ; and that it 

 will impart a greater degree of heat to the surface, the more rapidly 

 it increases the temperature of the strata situated below it, and 

 vice versd. 



Here again, what reasoning can merely indicate generally, may be 

 determined with the greatest precision by the aid of analytical 

 formulae ; and the assistance afforded by them, in this particular 

 case, is such, that it is now one and the same thing with geome- 

 tricians to know how much the heat increases in proportion as we 

 dig below the ground, and to ascertain the excess of temperature 

 which the central fire communicates to the surface ; the knowledge 

 of the one leads immediately to the knowledge of the other. Now 

 we can measure, as to each locality, the increase of temperature, 

 commencing from the surface ; we can thus also learn, for each 

 locality, the excess of temperature produced by the central heat. 



All the observations collected and discussed by the most learned 

 physiologists of our days, inform us, that the increase of temperature 

 in the strata lying immediately beneath the surface, is about a 

 degree in thirty metres, at a medium. In a globe of iron a similar 

 increase would only give a quarter of a centesimal degree, for the 

 actual elevation of the temperature of the surface. As a conse- 

 quence of the influence of the central fire, this elevation is very 



