130 IIAMBLES OF A NATURALIST. 



how rapidly it might be destroyed if a more or less 

 free outlet to the action of subterranean fires was 

 not always presented by the 550 volcanoes which 

 are disturbed over its surface like so many safety 

 valves.* 



the thickness and solidity of the earth's crust. The following figures 

 will furnish a more exact idea on this subject. 



The most superficial strata of the soil participate in all the varia- 

 tions of temperature which depend upon the seasons, and this 

 influence is exerted to a depth which, although it varies with the ati- 

 tude, is never very considerable. Beyond this point, the temperature 

 rises in proportion as we descend to greater depths, and it has been 

 shown by numerous and often repeated experiments that the increase 

 of temperature is on an average one degree (Fahrenheit) for about 

 every 54'5 feet. Hence it results that at a depth of about ?,000 yards 

 from the surface we should reach the temperature of boiling water. 

 In supposing that the heat increases in a uniform ratio, we should 

 have at a depth of twenty miles a temperature which would fuse 

 most of the silicates that enter into the composition of our rocks ; 

 and at a depth of about twelve miles from the surface we should be 

 on the verge of the incandescent mass which constitutes almost the 

 whole of our globe. 



Now this thickness, when compared with the dimensions of the 

 earth, would represent about one inch for a globe whose diameter 

 was about nine yards. In other terms, it would very nearly equal 

 the thickness of a sheet of ordinary letter paper for one of those 

 globes which are commonly used for geographical studies. With 

 this illustration of the question before us, we cease to be asfonished 

 at the movements which can agitate this pellicle ; and indeed if there 

 is anything to excite our surprise it is rather that the habitable 

 surface of our globe has not more frequently been the theatre of 

 such convulsions, which, although they might be terrible in their 

 results to ourselves, would not the less remain almost inappreci- 

 able on the "vast extent of our planet. 



* The following table gives the number and the geographical 

 distribution of all the volcanoes and solfataras (semi- extinct 

 volcanoes) whose existence has hitherto been verified. 



