SECT, xxvi.] CENTRAL HEAT OF THE EARTH. 281 



Should the earth's temperature increase at the rate of 1 

 for every 50 feet, it is clear that at the depth of 200 miles 

 the hardest substances must be in a state of fusion, and our 

 globe must in that case either be encompassed by a stratum 

 of melted lava at that depth, or it must be a ball of liquid 

 fire 7600 miles in diameter, inclosed in a thin coating of 

 solid matter ; for 200 miles are nothing when compared with 

 the size of the earth. No doubt the form of the earth, as 

 determined by the pendulum and arcs of the meridian, as 

 well as by the motions of the moon, indicates original fluidity 

 and subsequent consolidation and reduction of temperature 

 by radiation ; but, whether the law of increasing temperature 

 is uniform at still greater depths than those already attained 

 by man, it is impossible to say. At all events, internal 

 fluidity is not inconsistent with the present state of the earth's 

 surface, since earthy matter is as bad a conductor of caloric 

 as lava, which often retains its heat at a very little depth for 

 years after its surface is cool. Whatever the radiation of the 

 earth might have been in former times, certain it is that it 

 goes on very slowly in our days ; for M. Fourier has com- 

 puted that the central heat is decreasing from radiation by 

 only about the goroo^h part of a second in a century. If so, 

 there can be no doubt that it will ultimately be dissipated ; 

 but, as far as regards animal and vegetable life, it is of very 

 little consequence whether the centre of our planet be liquid 

 fire or ice, since its condition in either case could have no 

 sensible effect on the climate at its surface. The internal 

 fire does not even impart heat enough to melt the snow at 

 the poles, though so much nearer to the centre than any 

 other part of the globe. 



The immense extent of active volcanic fire is one of the' 

 causes of heat which must not be overlooked. 



The range of the Andes from Chili to the north of Mexico, 

 probably from Cape Horn to California, or even to New 

 Madrid in the United States, is one vast district of igneous 

 action, including the Caribbean Sea and the West Indian 



