VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 459 



Both the tremulous and wave-Hke motion observed in eartnquakes, may be 

 accounted for from such a vapour. In order to trace more particularly the man- 

 ner in which these two motions will be brought about, let us suppose the roof 

 over some subterraneous fire to fall in. If this should be the case, the earth, 

 stones, &c. of which it was composed, would immediately sink in the melted 

 matter of the fire below: hence all the water contained in the fissures and cavities 

 of the part falling in would come in contact with the fire, and be almost instantly 

 raised into vapour. From the first effbrt of this vapour, a cavity would be 

 formed (between the melted matter and superincumbent earth) filled with vapour 

 only, before any motion would be perceived at the surface of the earth : this 

 must necessarily happen, on account of the compressibility of all kinds of earth, 

 stones, &c. but as the compression of the materials immediately over the cavity, 

 would be more than sufficient to make them bear the weight of the superincum- 

 bent matter, this compression must be propagated on account of the elasticity of 

 the earth, in the same manner as a pulse is propagated through the air; and 

 again the materials immediately over the cavity, restoring themselves beyond 

 their natural bounds, a dilatation will succeed to the compression; and these two 

 following each other alternately for some time, a vibratory motion will be pro- 

 duced at the surface of the earth. If these alternate dilatations and compressions 

 should succeed each other at very small intervals, they would excite a like mo- 

 Trans. No. 454.] The Marquis of Worcester also, in his Century of Inventions, tells us that he 

 burst a cannon by the same means- 



It has been sometimes imagined that the vapours which occasion earthquakes were of the same 

 kind with those fulminating damps, of which we often meet with instances in coal mines. Now, 

 there are several things which make it very probable that this is not the case : it is true, the force of 

 such vapours is very great ; we have had instances where large beams of timber have been thrown 

 to the distance of 100 yards by them : (See Phil. Trans. No. 136) but what is this to the force of 

 that vapour which could throw stones of 20 or 30 tons weight to the distance of 3 leagues ? Nor in- 

 deed is it at all probable that any vapour, already in the form of a vapour, can, by suddenly taking fire, 

 increase its dimensions so much as to produce that immense quantity of motion which we observe in 

 some earthquakes: but this is rather to be expected from some solid body, such as water, which is ca- 

 pable of being converted, and that almost instantly, into one of the lightest, and perhaps one of the 

 most elastic, vapours in the world. Air, when heated to the greatest degree that it is capable of re- 

 ceiving from the hottest fires we can make, acquires a degree of elasticity about 5 times as great as that 

 of common air ; the vapour of gunpowder, while it is inflamed, has also about 5 times the elastic force 

 which it has when cold, (See Rob:ns's excellent tract on Gunnery.) Now if we suppose a fulminating 

 damp, of any kind, to increase its elasticity, when inflamed in the same proportion, this will be 

 abundantly sufficient to make it produce any eff^ects, which we have ever seen produced by any of 

 the damps of mines, &c. And indeed whoever carefully examines the effects, either of the damps of 

 mines, or of those fulminating damps that are raised from some rrietals, when in fusion, or when they 

 are dissolving in acids, will rather be inclined to think that the force of inflamed vapours is so far 

 from exceeding the proportion of 5 to one, that it falls considerably short of it, — Orig, 



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