THE EARTH 143 



acted like the glass of a hothouse and increased 

 the Sun's heat. 



The thickness of the Earth's crust is supposed 

 to be only about fifty miles, yet the tension of 

 the globe is so great that the whole mass is more 

 rigid than steel. The nature and condition of 

 the material within the crust is unknown, but it 

 is certainly intensely hot. Not only have we 

 volcanoes to prove this, but experiments with 

 borings show that the temperature of the Earth 

 increases about 1° F. for each 70 or 80 feet we 

 penetrate it. This fact would make us suppose 

 that the centre is fluid, but there are other facts 

 that make such fluidity doubtful. We do not 

 really know what would happen to hot matter at 

 the enormous pressure existing in the centre of 

 the Earth, and it is quite possible that at such 

 pressures even intensely hot matter might remain 

 solid, and melt only with reduction of the 

 pressure. 



Sartorius von Waltershausen and Hopkins 

 think that there "are no actual central fires, but 

 only internal seas of red-hot molten matter 

 scattered about in various parts of the inside of 

 our planet, situated not far from the surface of the 

 Earth, and separated from one another by masses 

 of solid strata." 



Attempts have often been made to estimate the 



