* EARTHQUAKES AND VOLCANOES. 



46. It lias been shown in our tract on Terrestrial Heat, that in 

 descending below the stratum of invariable temperature, the tem- 

 perature of the strata increases at the rate of about 1 for every 

 50 feet. The stratum of invariable temperature, at the mean 

 latitudes, being at a depth of less than 100 feet, and its tempera- 

 ture being nearly the same as the mean temperature at the surface ; 

 we may state in round numbers, that if the temperature at the 

 surface be taken to be 50, the temperature at the depth of one 

 mile will be 150, at the depth of two miles will be 250, and at 

 the depth of three miles will be 350. Now, water at the tempe- 

 rature of 250 produces steam bearing a pressure of 30 Ibs. per 

 square inch, being about half that of the steam generated in a 

 common locomotive, while water at 350 produces steam having a 

 pressure of 130 Ibs. per square inch, being more than twice the 

 pressure in a locomotive. 



47. The natural hot springs which exist in various parts of the 

 globe must issue from strata whose depth corresponds to their 

 temperature, rising through fissures or perforations in the superior 

 strata, produced by disruptions effected by the pressure from 

 within prevailing over the tenacity of the materials composing such 

 strata. If the temperature of the water issuing from such springs 

 could be taken to be that which it had in the reservoir from which 

 it has risen, such temperature would supply at least an approxi- 

 mate index of the depth of such reservoir. But it must be 

 considered, that in rising to the surface it passes through a 

 succession of strata of constantly decreasing temperature, com- 

 posed of materials of various conducting powers and capacities for 

 heat, and that in its ascent it must part with more or less of its 

 heat, and therefore that its temperature on issuing from the spring 

 at the surface must be less than that of the subterranean reservoir 

 from which it has risen. How much must, [in each case, be 

 allowed for this loss of temperature is a problem of considerable 

 difficulty, and one which at best, with our present data, admits of 

 no more than a rough approximative solution. 



48. Observations made very extensively upon thermal springs, 

 prove them to be completely independent of the strata from and 

 under which they rise. Neither do they prevail exclusively in 

 volcanic regions. Indeed, Humboldt affirms that the hottest per- 

 manent springs hitherto discovered are some found by himself, at 

 a great distance from any volcano, as for example, the "Aguas 

 Calientes de las Trincheras," between Puerto Cabello and New 

 Valencia, in Venezuela, South America, the temperature of which 

 was 194^, and which issued through a stratum of granite; and 

 the " Aguas Comangillas," near Guanaxuato, in Mexico, of which 

 the temperature was 205|, being only 6| below the boiling point. 



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