THE EARTH. 



31 



quakes, such as the tremulous, the pulsative, 

 the perpendicular, and the inclined ; but 

 these are rather the distinctions of art than 

 of nature, mere accidental differences arising 

 from the situation of the country or of the 

 cause. If, for instance, the confined h're acts 

 directly under a province or a town, it will 

 heave the earth perpendicularly upward, 

 and produce a perpendicular earthquake. If 

 it acts at a distance, it will raise that tract 

 obliquely, and thus the inhabitants will per- 

 ceive an inclined one. 



Nor does it seem to me that there is much 

 greater reason for Mr. BufFon's distinction of 

 earthquakes ; one kind of which he supposes" 

 to be produced by fire in the manner of vol- 

 canoes, and confined but to 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 endeavour- 

 ing to get free. For how do these two causes 

 differ? Fire is an agent of no power whatso- 

 ever without air. It is the air, which being at 

 first compressed, and (hen 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 bonds 

 of the earth, and acquiring elasticity by heat. 

 that produces all those appearances which 

 are generally ascribed to the operation of 

 fire. When, therefore, we are told lh;tt there 

 are two causes of earthquakes, we only learn 

 that a greater or smaller quantity of heat pro- 

 duces 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 pro- 

 ducing 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 experience tell us that 

 the earth is full of inflammable materials, and 

 tint fires are produced wherever we descend ; 

 although it tells us that those countries where 

 there are volcanoes, are most subject to earth- 

 quakes; 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 



* Buffbn, vol. ii. p. 328. 

 io. 4. 



long calculations, give theory an air of de- 

 monstration. Mr. Amontons b has been par- 

 ticularly 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 amazing enough, however, 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 

 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 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 seventieth part of the way. The 

 air, therefore, at the centre, must be infinitely 

 heavier than mercury, or any body that we 

 know of. This granted, we shall take some- 

 thing 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 degree of boiling water, (as we 

 have proved that the density of the air is here 

 very great.) its elasticity must be in propor- 

 tion ; 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, and, in short, be per- 

 fectly 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) destroy the solar system, and 

 even the fixed stars themselves." These 

 reveries generally produce nothing; for, as I 

 have ever observed, increased calculations, 

 while they seem to tire the memory, give the 

 reasoning faculty perfect repose. 



However, as earthquakes are the most 

 formidable ministers of nature, it is not to be 

 wondered that a multitude of writers have 

 been curiously employed in their considera- 

 tion. Woodward has ascribed the cause to 

 a stoppage of the waters below the earth's 

 surface by some accident. These being thus 

 accumulated, ami yet acted upon by fires. 



b Memoires de 1'Academie de Sciences. An. 1703. 

 K 



