218 COSMOS. 



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

 at which the invariability of the temperature begins in the 

 temperate 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 calientes 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 former 

 of these, which issued from granite, had a temperature of 194'5 

 the latter, issuing from basalt, 205 0> 5. The depth of the source 

 from whence the -water flowed with this temperature, judging 

 from what we know of the law of the increase of heat in the 

 interior of the earth, \vas probably 7140 feet, or above two miles. 

 If the universally diffused terrestrial heat be the cause of ther- 

 mal springs, as of active volcanoes, the rocks can only exert an 

 influence by their different capacities for heat and by their 

 conducting powers. The hottest of all permanent springs 

 (between 203 and 209) are likewise in a most remarkable 



* The profound investigations of Boussingault fully convince me that 

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

 exactly corresponds with the mean temperature of the air. The follow- 

 ing instances are sufficient to illustrate this fact : 



Temperature at i 



Stations within Tropical French foot [1-006 of Mean Temperature Height, in English feet, 

 ZoneSe the English foot] below of the air j above the level of the sea. 



the earth's surface. 



Guayaquil 78'8 ... 78'1 ... 



Anserma Nuevo ... 74'6 ... 74'8 ... 3444 



Zupia 70-7 ... 70'7 ... 4018 



Popayan 64*7 .. 65'6 ... 5929 



Quito 59-9 ... 59-9 ... 9559 



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

 which I am probably in some degree the cause by my observations on 

 the Cave of Caripe (Cueva del Guacharo), (Rel. hist., t. iii. pp. 191- 

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

 mean temperature of the air of the convent of Caripe, 65'3, not with 

 the temperature of the air of the cave, 65'6, but with the temperature 

 of the subterranean stream, 62 0- 3; although I observed (Rel. hist., t. iii. 

 pp. 146 and 194,) that mountain water from a great height might pro- 

 bably be mixed with the water of the cave. 



