rnOOEEDINGS OF THE POLYTECHNIC ASSOCIATION. 785 



our present existing continents and islands," and, indeed, tliey have 

 been at work long enough to have, if unopposed, produced this vf3ry 

 result. 



It is the earthquake and the volcano which places themselves in 

 opposition to this destructive tendency ; so that we may regard the 

 igneous agents as in constant antagonism to the aqueous agents, the 

 latter laboring incessantly to obliterate the land, while the forr«:ier are 

 equally active in restoring it. What are these igneous agents, and 

 what is their source ? 



It is a fact perfectly assured that, in proportion as we descend into 

 the earth, the heat augments, and the deeper we go the hotter the 

 earth is found to be. This is proven by numberless observations 

 that have been made, not only in the temperature of the air in mines, 

 but in that of rocks and in the water issuing from them. In boring 

 artesian wells the water always comes up hot, and the deeper the 

 boring the hotter the water. In the famous well in Paris at La 

 Grenelle, the water rises from a depth of 1,794 feet, and its tempera- 

 ture is eighty-two degrees Fahrenheit, which is almost that of the 

 equator. The same thing appears in natural hot springs, as for 

 instance that of Arkansas, which is scalding hot, and shows a con- 

 tinuous temperature of 180 degrees. This increase is estimated at 

 about a degree of the thermometer additional warmth for every ninety 

 feet of additional depth ; or about fifty-eighty degrees per mile, 

 " At twenty miles depth," says a distinguished physicist, " according, 

 to this rate, the ground must be fully red hot ; and at no such verj 

 great depth beyond, either the wJiole must be melted, or only the 

 most infusible and intractable kinds of material, such as our fire-clays 

 and fiints would prevent some degree of solidity." 



Now, though geology does not say that there may not be a solid 

 central mass in the interior of the earth, " kept solid in spite of the 

 heat by the enormous pressure," it does say that an immense range 

 of terrestrial phenomena compels us to conclude that beneath the 

 crust of the earth there is a sea of liquid fire, on which the continents 

 and the land underneath the ocean are floating. This central fire is 

 not only incandescent matter, but it is matter in a state of energetic 

 elasticity, continually reacting upon the structure of the earth, and 

 making itself felt more or less palpably ; sometimes producing violent 

 undulatory motions, and at otlier times breaking through the crust 

 and vomiting forth lava and the central fluid. The former of these 

 commotions are styled earthquakes ; the latter, volcanoes : there is^ 



[IxsT.] 60 



