208 REACTION OF THE INTERIOR OF THE EARTH 



companions in Northern Asia, that the mixture of the waters 

 from the various sources from which springs originate 

 mountains, hills, or deep subterranean strata makes it very 

 difficult to determine the position of the " Isogeothermal 

 lines" ( 201 ) (lines of equal internal terrestrial heat), from the 

 temperature of water as it issues from the earth. The tem- 

 perature of springs, which, for the last half century, has been 

 so much an object of physical research, depends, indeed, like 

 the limit of perpetual snow, on the concurrent influence of 

 many and very complicated causes. It is a function of the 

 temperature of the stratum in which they take their rise, of 

 the specific heat of the soil, and also of the quantity and tem- 

 perature of the water which falls, in rain, snow, or hail, ( 203 ) 

 and which, from the conditions of its origin, has a different 

 temperature from that of the air in the lower portion of the 

 atmosphere. ( 203 ) In order that cold springs may shew the 

 true mean temperature of the place where they issue from the 

 ground, they must be unmixed with waters coming either from 

 great depths, or from mountain elevations; and they must have 

 passed through a long subterranean course, at a depth below 

 the surface of about forty to sixty feet in our latitudes, 

 and, according to Boussingault, of one French foot within 

 the tropics : ( 204 ) these depths being those at which the 

 temperature is supposed to be constant, or unaffected by the 

 horary, diurnal, or monthly variations of the temperature of 

 the atmosphere. 



Hot springs issue from rocks of every kind; the 

 hottest permanent springs yet known are those found by 

 myself, at a distance from any volcano, the " Aquas ca- 

 lientes de las Trincheras," in South America, between 

 Porto Cabello and New Valencia, and the "Aquas de 



