92 HISTORY OF 



produced by fire, in the manner of volcanoes, 

 and confined to but a very narrow circumference. 

 The other kind he ascribes to the struggles of 

 confined air, expanded by 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 air 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 generally 

 ascribed to the operation of fire. When, there- 

 fore, we are told 'that there are two causes of 

 earthquakes, we only learn, that a greater or 

 smaller quantity of heat produces those terrible ef- 

 fects ; 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 effects as they could ; and, magnifying its 

 powers, have called in but a very moderate degree 

 of heat to put it in action. Although experience 

 tells us that the earth is full of inflammable mate- 

 rials, and that fires are produced wherever we de- 

 scend ; although it tells us that those countries 

 where there are volcanoes, are most subject to 

 earthquakes, yet they step out of the way, and 

 so find a new solution. These only allow but 

 just heat enough to produce the most dreadful 

 phenomena, and backing their assertions with 

 long calculations, give theory an air of demon- 



