THERMAL SPRIGS. 185 



tion. The temperature of the soil is not the same at equal 

 elevations in the entire range of the Alps as the isothermal 

 surfaces, which unite the points of the same average temper- 

 ature of springs, rise higher above the level of the sea, inde- 

 pcndently of the injluence of latitude, in proportion to the av- 

 erage convexity of the surrounding soil ; perfectly in accord- 

 ance with the laws of the distribution of heat in a solid body 

 of varying thickness, with which the relief (the mass-eleva- 

 tion) of the Alps may be compared.'* 



In the chain of the Andes, and indeed iil those volcanic 

 parts of it which present the greatest elevations, the burying 

 of thermometers may, in particular cases, lead to deceptive 

 results by the influence of local circumstances. Fronsi the 

 opinion formerly held by me, that black, rocky ridges, visible 

 at a great distance, which penetrate the snowy region, are 

 not always indebted for their entire freedom from snow to 

 the steepness of their sides, but to other causes, I buried the 

 bulb of a thermometer only three inches deep in the sand, 

 which filled the fissure in a ridge on the Chimborazo at an 

 elevation of 18,290 feet, and therefore 3570 feet above the 

 summit of Mont Blanc. The thermometer permanently 

 showed 10°-5 F. above th|fe freezing-point, while the air was 

 only 4P'o F. above that point. The result of this observa- 

 tion is of some importance ; for even 2558 feet lower, at the 

 lower limit of perpetual snow of the volcano of Quito, ac- 

 cording to numerous observations collected by Boussingault 

 and myself, the average temperature of the atmosphere is 

 not higher than 34°-9 F. The ground temperature of 42°-5 

 must, therefore, be ascribed to the subterranean heat of the 

 doleritic mountain — I do not say of the entire mass, but to 

 the currents of air ascending in it from the depths. At the 

 foot of Chimborazo, at an elevation of 948G feet toward the 

 hamlet of Calpi, there is, moreover, a small crater of erup- 

 tion, Yana-Urcu, which, as indeed is shown by its black, 

 slag-like rock (augitic porphyry), appears to have been act- 

 ive in the middle of the fifteenth century.* 



The aridity of the plain from which Chimborazo rises, and 

 the subterranean brook, which is heard rushing under the 

 volcanic hill (Yana-Urcu) just mentioned, have led Boussin- 

 gault and myselff at very different times to the idea that 

 the water which the enormous masses of snow produce daily 

 by melting at their lower limit sinks into the depths through 



* Humboldt, Kldriere Schriften, bd. i., p. 139 and 147. 

 , f Humboldt, Op. cit., s. 140 and 203. 



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