THE EARTH. 79 



supposes* to be produced by fire in the manner of volcanoes, 

 and confined but to a very narrow circumference. The othei 

 kind he ascribes to the struggles of confined air, expanded bj 

 heat in the bowels of the earth, and endeavouring to get free. 

 For how do these two causes differ ? Fire is an agent of no 

 power whatsoever without air. It is the air, which being at 

 first compressed, and then dilated in a cannon, that drives the 

 ball with such force. It is the iair struggling for vent in a vol- 

 cano, that throws up its contents to such vast heights. In short, 

 it is the air confined in the bowels of the earth, and acquiring 

 elasticity by heat, that produces all those appearances which are 

 generaliy ascribed to the operation of fire. When, therefore, 

 we are told that there are two causes of earthquakes, we ovly 

 learn that a greater or smaller quantity of heat produces those 

 lerrible effects ; for air is the only active operator in either. 



Some philosophers, however, have been willing to give the 

 air as great a share in producing these terrible efforts as they 

 could ; and, magnifying its powers, have called in but a very 

 moderate degree of heat to put it in action. Although experi- 

 ence teU us that the earth is full of imflammable materials, and 

 that fires are produced wherever we descend ; although it tells us 

 that those countries where there are volcanoes, are most subject 

 to earthquakes ; yet they step out of their way, and so find a 

 new solution. These only allow but just heat enough to pro- 

 duce the most dreadful phenomena, and, backing their assertions 

 with long calculations, give theory an air of demonstration. IMr 

 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 e:-pansion. 



It is amusing enough, ho^rever, to trace the progress of a 

 philosophical fancy let loose in imaginary speculations. They 

 run thus : " A very moderate degree of heat may bring the aii 

 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 surface of the earth becomes almost as 

 heavy as quicksilver. This, however, is but a very slight depth 

 in comparison of the distance to the centre, and is scarcely a 



2 Bufibn, vol. ii. p. 32S. 3 Memoires da I'Acadamie de Scieuccs. An. 17(2. 



