LETTER VII. 



VOLCANOES AND EARTHQUAKES. 



In my last letters I have given you numerous 

 and well-marked proofs of the mighty effects of 

 elastic vapours and gases at work at but moderate 

 depths below the surface of the earth. You will 

 now be prepared to consider it probable, if not 

 quite certain, that at greater depths this expansive 

 power must be enormously increased, in conse- 

 quence of the proportional increase of heat. And 

 you will readily conceive that this upheaving 

 power may at last become so great, as to counter- 

 balance the pressure of thousands of atmospheres, 

 or even the huge weight of the strata of the earth's 

 crust. 



I have before shown, that, at a certain depth, 

 which may be taken at about forty thousand feet, 

 an expansive force must be given to the vapour of 

 water, strong enough to enable it to upheave the 

 whole liquid column that presses on it. I showed 

 too, that whatever water may find its way to a 

 depth greater than this, must be always in a boiling 

 state ; that is, that the heat which it takes up from 

 the earth must immediately turn it into steam. If, 

 however, the steam given off under these circum- 



