266 Conjectures concerning the Cause, and Observations 
over some’subterraneous fire to fallin. If this should be the case, 
the earth, stones, &c. of which it was composed, would imme- 
whvever carefully examines the effects, either of the damps of mines, or of 
those fulminating damps, that are raised froin some metals, 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 
five to one, that it falls considerably short of it. 
But though we should suppose that this proportion holds good, where 
shail we find a place capable of containing a sufficient quantity of such a 
vapour, to produce the great effects of earthquakes? It will be said, per- 
haps, in subterraneous caverns. To this we may answer, that he, who is 
but moderately acquainted with the structure of the earth, and the ma- 
terials of which it is composed, will be little inclined to allow of any great 
or extensive caverns in it(/). But, though this should be admitted, how 
can it come to pass that these caverns should not be filled with water? If 
it is alleged, that the water is expelled, as the vapour is formed, why should 
not the vapour, as it is supposed to be the lighter, be expelled, rather than 
the water, by the same passages by which the water is to be expelled? But 
let us suppose this difficulty also to be got over, and the water to be re- 
moved, aud we shall then have a gage for the density of the vapour; for 
it must be just sufficient to make it capable of sustaining a column of wa- 
ter, whose height is equal to that of the surface of the sea above the bot- 
tom of the cavern, in which the vapour is supposed tv be contained. Now, 
since the mean weight of earth, stones, &c. is not less than two and a half 
times the weight of water, this vapour must be increased to two and a half 
times its original elasticity, before it can, in any wise, raise the earth above 
it; and if we suppose it to be increased to five times its original elasticity, 
it will then be no more thar twice able to do so; in which case, so much 
vapour only can be discharged from the cavern, to produce an earthquake, 
as is equal to the content of the cavern: and what must the size of that 
cavern be, which could contain vapour enough to produce the earthquake 
of the ist of November 1755, in which an extent of earth of near three 
thousand miles diameter was considerably moved? or how can we suppose, 
that the roof of sucha cavern, when so violently shaken, should avoid falling 
in? especially, as itis hardly to be supposed, that any inflamed vapour what- 
soever should be able to move the earth over these caverns, if they lay at 
auy great depth, since the weight of less than three miles depth of earth is 
capable of retaining the inflamed vapour of gunpowder within the original 
dimensions of the gunpowder itself ; and common air, compressed by the 
same weight (supposing the known law of its compression to hold so far), 
would be of greater density than water. $ 
We may ask sull further, whence such vast quantities of vapour should 
be formed, or what sources they must be, which would not be exhausted (if 
they were not again replenished) by a very few repetitions of such im- 
mense discharges. — 
(1) In this passage, we see another instance of Mr. Michell’s happy sa- 
gacity, in rejecting the notion of extensive Caverns within the Earth, long 
before the observations of Maskelyne and calculations of Hutton, (Phil. 
Mag. vol. xxxviii. p. 112) had established a specific gravity for the whole 
Globe, quite inconsistent with such a puerile supposition: the confirmatory 
experiments however of Cavendish, and the sublime deductions of Laplace, 
in further confirmation of the same well-established fact, are yet unable to 
shake the belief of numbers of Plutonists and Geognosts, in the dogma 
common to both their Creeds, viz. that Granite, of their very precise hand- 
specimen kind, occupies all the central parts of the Globe! 
diately 
