THE EARTH. 93 



stration. M. Amontons* has been particularly 

 sparing of the internal heat in this respect ; and 

 has shown, perhaps accurately enough, that a very 

 moderate degree of heat may suffice to give the 

 air amazing powers of expansion. 



It is wonderful, however, to trace the pro- 

 gress of a philosophical fancy let loose in imagi- 

 nary speculations. They run thus : " A very mo- 

 derate degree of heat may bring the air into a 

 condition capable of producing earthquakes ; for 

 the air at the depth of forty-three thousand five 

 hundred and twenty-eight fathom below the sur- 

 face of the earth, becomes almost as heavy as 

 quicksilver. This, however, is but a very slight 

 depth in comparison of the distance to the cen- 

 tre, and is scarcely a seventieth part of the way. 

 The air, therefore, at the centre, must be infinite- 

 ly heavier than mercury, or any body that we 

 know of. This granted, we shall take something 

 more, and say, that it is very probable there is 

 nothing but air at the centre. Now let us sup- 

 pose this air heated, by some means, even to the 

 degree of boiling water, as we have proved that 

 the density of the air is here very great, its elasti- 

 city must be in proportion : a heat, therefore, 

 which at the surface of the earth would have pro- 

 duced but a slight expansive force, must at the 

 centre produce one very extraordinary, and, in 

 short, be perfectly irresistible. Hence this force 

 may, with great ease, produce earthquakes and 

 if increased, it may convulse the globe ; it may, 

 by only adding figures enough to the calculation, 



* Memoires de 1' Academic cles Sciences, an. 1703. 



