THERMAL SPRINGS. 195 



\Vlien a low plain is compared with a mountain chain or 

 plateau, rising boldly to a height of many thousand feet, the 

 law of the increase and diminution of temperature does not 

 depend simply upon the relative vertical elevation of two 

 points on the earth's surface (in the plain and on the summit 

 of the mountain). If we should calculate from the supposi- 

 tion of a definite proportion in the change of temperature in 

 a certain number of feet from the plain upwards to the sum- 

 mit, or from the summit downwards to the stratum in the 

 interior of the mountain mass which lies at the same level 

 as the surface of the plain, we should in the one case find the 

 summit too cold, and in the other the stratum in the in- 

 terior of the mountain far too hot. The distribution of 

 heat in a gradually sloping mountain (an undulation of the 

 surface of the earth) is dependent, as has already been re- 

 marked, upon form, mass, and conductibility ; upon insola- 

 tion, and radiation of heat towards the clear or cloudy 

 strata of the atmosphere ; and upon the contact and play 

 of the ascending and descending currents of air. According 

 to such assumptions, mountain springs must be very abun- 

 dant, even at very moderate elevations of four or five ^hou- 

 sand feet, where the temperature would exceed the average 

 temperature of the locality by 72 or 90 degrees ; and how 

 would it be at the foot of mountains under the tropics, 

 which at an elevation of 14,900 feet are still free from 

 perpetual snow ; and often exhibit no volcanic rock, but 

 only gneiss and mica schist I 44 The great mathematician, 

 Fourier, who had been much interested in the fact of the 

 volcano of Jorullo having been upheaved, in a plain, where 

 for many thousands of square miles around no unusual ter- 

 restrial heat was to be detected, occupied himself at my 

 request in the very year before his death with theoretical 

 investigations upon the question, how in the elevation of 

 mountains and alterations in the surface of the earth, the 

 isothermal surfaces are brought into equilibrium with the 

 new form of the soil. The lateral radiation from strata 

 which lie in the same level, but are differently covered, 

 44 I differ here from the opinion of one of my best friends, a physi- 

 cist \vhohas done excellent Rervice as regards the distribution of telluric 

 heat. See " upon the cause of the hot springs of Leuck and Warm- 

 brunn," Bischof, Lehrbuch der chtfMMchcn und physikalisclien 

 Bd. i, s. 127133. 



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