ON ITS EXTERIOR. THERMAL SPRINGS. 193 



the Alps ; for, apart from the influence of geographical 

 latitude, the elevation of the isothermal planes, con- 

 necting the points at which the springs have the same 

 mean temperature, increases with the increasing mean 

 elevation of the surrounding country: all in accordance 

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

 of varying thickness, to which the relief (or mass ele- 

 vation) of the Alps may be compared." 



In the Andes, and precisely in that volcanic portion 

 of the chain which presents the greatest elevations, the 

 sinking of thermometers in the ground may in some 

 cases, under the influence of particular circumstances, 

 give rise to delusive results. In pursuance of a pre- 

 viously formed opinion, that the entire absence of snow 

 on the dark ridges of black rock, seen from afar inter- 

 secting the snowy region, may not always be exclusively 

 due to the configuration and steepness of their sides, 

 but may also be in part attributable to other causes, 

 I plunged the bulb of a thermometer only three or four 

 inches into the sand, which filled the cleft in such a 

 ridge on the Chimborazo, at an elevation of 18,288 

 feet, or 2550 feet higher than the summit of Mont 

 Blanc. The thermometer showed persistently 42 -4 

 Fahr., while the external air was only 36*9. The result 

 of this observation is of some importance, because 2560 

 feet lower down, at the lower limit of perpetual snow 

 on the volcanoes of Quito, the mean temperature of the 

 air, inferred from a large number of observations col- 

 lected by Boussingault and myself, is under 35. The 

 temperature of 42-4 must, therefore, have been due to 

 the subterranean warmth of the dolerite mountain : 

 I do not mean to say to the temperature of the entire 



YOL. iv. O 



