48 A HISTORY OF 



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 volcano, 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 gene- 

 rally ascribed to the operation of fire. When, therefore, we are told 

 that there are two causes of earthquakes, we only learn that a greater 

 or smaller quantity of heat produces those terrible 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 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 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 produce the most dreadful phenomena, and backing their 

 assertions with long calculations, give theory an air of demonstration 

 Mr. 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 amusing enough, however, to trace the progress of a philo- 

 sophical fancy let loose in imaginary speculations. They run thus : 

 " A very moderate degree of heat may bring the air into a condition- 

 capable of producing earthquakes ; for the air, at the depth of torty- 

 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 centrf ; and is scarce a seventieth part of the way. The air, there- 

 fore, at the centre, must be infinitely 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 suppose this air heated, by some means, even to the de- 

 gree of boiling water, as we have proved that the density of the air is 

 here very great, its elasticity must be in proportion ; a heat, therefore, 

 which at the surface of the earth would have produced but a slight 

 expansive force, must, at the centre, produce one very extraordinary, 

 ana, K short, be perfectly irresistible. Hence this force may, wilh 

 great ease, produce earthquakes ; and, if increased, it may convulse 

 the globe ; it may (by only adding figures enough to the calculation) 

 destroy the solar system, and even the fixed stars themselves." These 

 reveries generally produce nothing ; for, as I have ever observed, in- 

 creased calculations, while they seem to tire the memory, give the 

 reasoning faculty perfect repose. 



However, as earthquakes are the most formidable ministers of na- 

 ture, it is not to be wondered that a multitude of writers have been 



* Memoires de 1' Academic des Sciences. An. 1703. 



