Hot mtd Tliermal Springs. 14iS 



cited in Chap. XVIII, No. 1, 8, 12, and 14, wliich shew year- 

 ly variations of temperature, but have yet a higher mean tem- 

 perature than that of the place where they rise. In general, 

 all thermal springs which shew yearly variations of tempera- 

 ture, may be considered as proofs of this assertion. 



It must be remembered that the limit of the influence of the 

 external temperature is absolute, but not so that of the internal. 

 According as the depth below the surface be greater or less, so 

 will it require a longer or shorter time after the minimum tem- 

 perature of the air, before the temperature of the variable crusts 

 of the earth will begin constantly to increase, until the tempe- 

 rature of the air reach its maximum. And according as the 

 depth below the surface be greater or less, so will it require a 

 longer or shorter time after that maximum, before the tempe- 

 rature will begin to decrease again, which will continue until 

 the return of the former period. Thus, during the first half 

 year, there wiU be a transmission of heat from the surface to- 

 wards the interior ; and during the second half-yeai', on the 

 contrary, from the interior towards the surface. This os- 

 cillation dui-ing these two periods can, of course, only take 

 place within the variable crust of the earth. Should ever the pe- 

 riod from the minimum to the maximum increase to the double, 

 the depth to which the external temperature penetrates would 

 also increase almost to the double. That, in fact, the maxi- 

 mum and minimum temperatures of the air do not coincide with 

 those of the soil ; but that the yearly extremes of the latter oc- 

 cur, at the deepest points of the variable crust of the earth, 

 according to the observations on the temperature of springs, 

 about three months later than those of the former, is evidently 

 a consequence of the slow transmission of the heat through the 

 strata of earth and rocks. This circumstance, however, has no 

 effect upon our considerations. 



But how is it with the heat proceeding from the interior ? 

 Here we have no changes of the seasons. Here there is always 

 an excess of heat in the interior, and a smaller quantity in the 

 variable parts of the earth's crust. The direction of the com- 

 munication of the heat must, therefore, always be the same ; 

 namely, from the interior towards the surface. In the part of 

 the earth's crust, subject to variations of temperature, there is. 



