HEAT. 699 



Mallet, from this and other data, calculates, that 7,200 cubic miles of 

 crushed rock would cause heat enough to make all the volcanic moun- 

 tains of the globe ; and, as the ejections of the volcanoes have been 

 going forward through a very long period, the action would require 

 but an infinitesimal amount of annual crushing, — not over 0*606 of a 

 cubic mile. Whether Mallet's conclusion, " that the crushing of the 

 earth's solid crust affords a supply of energy sufficient to account for 

 terrestrial volcanicity," is correct or not, the fact is well established, 

 that motion in the earth's rocks has been a powerful source of heat. 



3. Internal Heat. — The proofs of the existence of a source of heat 

 within the earth are the following : — 



1. The spheroidal form of the earth (p. 9), this being evidence that 

 the earth was originally fluid. 



2. Borings for Artesian wells and shafts in mines have afforded a 

 means of taking the temperature of the earth at different depths ; and 

 it has been uniformly found that, after passing the limit of surface- 

 action, the heat increases. The ordinary rate is 1° F. for 50 or 60 

 feet of descent. At the Artesian well of Grenelle, a temperature of 

 85° F. was obtained at 2,000 feet, equivalent to 1° F. for every 60 feet 

 of descent. In Westphalia, at Neusalzwerk, in a well 2,200 feet deep, 

 the temperature at the bottom was 91° F., or 1° F. for 50 feet of de- 

 scent. At Pregny, near Geneva, a depth of 680 feet gave 63° F. At 

 Yakutsk, Siberia, Magnus found a gain of 15° F. in descending 407 

 feet, equal to 1° F. for 27 feet. The variations are considerable ; but 

 still the facts authorize the ratio above given. 



It has been proposed to make a tropical climate in the Garden of 

 Plants, by taking the heat from the earth's interior. Arago and Wal- 

 ferdin have estimated that, at a depth of 3,000 feet, the water would 

 have a temperature of 200° F., " sufficient not only to cheer the tropical 

 birds and monkeys of the Zoological Gardens, and the hot-houses and 

 green -houses of the establishment, but to give warm baths to the in- 

 habitants of Paris." 



The rate 1° F. for fifty feet of descent, in the latitude of New York, 

 would give heat enough to boil water, at a depth of 8,100 feet ; and 

 3,000° F., the fusing-point of iron, at a depth of about twenty-eight 

 miles. As the ratio, however, cannot be an arithmetical one, because 

 of both the greater conductivity of the earth below (owing to greater 

 density) and the increased pressure, the depth of fusion exceeds this 

 amount ; but how much, has not yet been determined. 



The amount of heat now lost by the earth, as a consequence of 

 cooling, is, according to Thomson, such as would melt annually a com- 

 plete covering of ice, # 0085 millimeters thick, to water at 32° F., or 

 bring 777 cubic miles of ice to the same state. 



