upon the Phenomena, of Earthquakes. 265 
of these volcanos, and whenever this happens, it will be imme- 
diately raised by the heat of them into a vapour, whose elastic 
force is capable of producing the most violent effects*. 
56. Both the tremulous and wave-like motion observed in 
earthquakes, may be very well accounted for from such a vapour. 
In order to trace a little more particularly the manner in which 
these two motions will be brought about, let us suppose the roof 
over 
* There are many effects produced by the vapour of water, when in- 
tensely heated, which make it probable, that the force of gunpowder is not 
near equal to it. The effects of an exceeding small quantity of water, upon 
which melted metals are accidentally poured, are such, as, I think, could 
in no wise be expected from the like quantity of gunpowder. Founders, 
if they are not careful, often experience these effects to their cost. An 
accident of this kind happened about forty years since, at the casting of 
two brass cannon at Windmill-hill, Moorfields. “ The heat of the metal 
of the first gun drove so much damp into the mould of the second, which was 
near it, that as soon as the metal! was let into it, it blew up with the greatest 
violence, tearing up the ground some feet deep, breaking down the fur- 
nace, untiling the house, killing many spectators on the spot, with the 
streams of melted metal, and scalding many others in-a most miserable 
manner.” [See the note at the end of process 44th of the English transla- 
tion of Cramer’s Art of Assaying Metals. ] 
Other instances of the violence of vapours raised from water, are fre- 
quently to be met with: one of Papin’s digesters being placed between the 
bars of a grate, where there was a fire, was, after some time, burst by the 
violence of the steam, the fire was all blown out of the grate, and a piece 
of the digester was driven against the leaf of a strong oak table, which it 
broke to pieces. [See Phil. Trans. No. 454, or Martyn’s Abr. vol. viii. 
p. 465.] ‘The marquis of Worcester also, in bis Century of Inventions, 
tells us, that he burst a cannon by the same means. 
It has been sometimes imagined, that the vapours, which occasion earth- 
quakes, 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 vapous is very great, we have bad instances, where large beams of 
timber have been thrown to the distance of an hundred yards by them: 
[see Philos. Trans. No, 156, or vol. ii, p. 381. Lowthorp’s Abr.] but what 
is this to the force of that vapour, which could throw stones of twenty 
or thirty ton weight to the distance of three leagues? Nor, indeed, is 1t 
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 
capable 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 receiving from the hottest 
fires we can make, acquires a degree of elasticity about five times as great 
as that of common air: the vapour of gunpowder, whilst it is inflamed, has 
also about five times the elastic force which it has when cold. [See Robins’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 effects, which we 
have ever seen produced by any of the damps of mines, &c. And, ee 
whoever 
