HOT SPRINGS. 



221 



perature when they are unmixed with the waters rising from 

 great depths, or descending from considerable mountain eleva- 

 tions, and when they have passed through a long course at a 

 depth from the surface of the earth which is equal in our lati- 

 tudes to 40 or 60 feet, and, according to Boussingault, to about 

 one foot in the equinoctial regions ;* these being the depths at 

 which the invariability of the temperature begins in th^ tem- 

 perate and torrid zones, that is to say, the depths at which 

 horary, diurnal, and monthly changes of heat in the atmosphere 

 cease to be perceived. 



Hot springs issue from the most various kinds of rocks. The 

 hottest permanent springs that have hitherto been observed 

 are, as my own researches confirm, at a distance from all vol- 

 canoes. I will here advert to a notice in my journal of the 

 Aguas Calie7ites de las Trincheras, in South America, between 

 Porto Cabello and Nueva Valencia, and the Aguas de Coman- 

 gillas, in the Mexican territory, near Guanaxuato ; the for- 

 mer of these, which issued from granite, had a temperature of 

 194°-5; the latter, issuing from basalt, 205°-5. The depth 

 )f the source from whence the water flowed with this temper- 

 ature, judging from v/hat we know of the law of the increase 

 of heat in the interior of the earth, was probably 7140 feet, 

 or above two miles. If the universally-difilised terrestrial 

 heat be the cause of thermal springs, as of active volcanoes, 

 the rocks can only exert an influence by their difTerent capaci- 



" The profound investigations of Boussingault fully convince me, that 

 in the tropics, tlie temperature of the ground, at a very slight depth, ex 

 actly corresponds witli the mean temperature of the air. The follow 



ing uistauces are sufficient to ilhistrate this fact : 



The doubts about the temperature of the earth within the tropics, of 

 wliich I am probably, in some degree, the cause, by my observations 

 oh the Cave of Caripe (Cueva del Guacharo^), Rel. Hist., t. ii:., p. 191- 

 196), are resolved by the consideration that I compared the presumed 

 mean temperature of the air of the convent of Caripe, G5°*3, not with 

 the temperature of the air of the cave, G5°-G, but vs'ith tlje temperature 

 of the subterranean stream, G2'^*3, although I ob.served {Rel. Hist., t 

 iii., p. 146 and 194) that mountain water from a gre.it heigh 

 probably be mixed wilh the water of the cave 



t might 



