C 596 ] 
for from fucli a vapour. In order to trace a little 
more particularly the manner in which thefe two 
motions 
gunpowder, whilft it is inflamed, has alfo about five times the 
elaftic force which it has when cold. [See Robins’s excellent 
tract on Gunnery.] Now, if we fuppofe a fulminating damp, of 
any kind, to increafe its elafticity, when inflamed in the fame pro- 
portion, this will be abundantly fufficient to make it produce any 
effects, which we have ever feen produced by any of the damps of 
mines, l5c. And, indeed, whoever carefully examines the eft'edls, 
either of the damps of mines, or of thofe fulminating damps, that 
are raifed from fome metals, when in fufion, or when they are 
diffolving in acids, will rather be inclined to think, that the force 
of inflamed vapours is fo far from exceeding the proportion of five 
to one, that it falls confiderably fhort of it. 
But though we fhould fuppofe that this proportion holds good, 
where {hall we find a place capable of containing a fufficient quan- 
tity of fuch a vapour, to produce the great effetfts of earthquakes ? 
It will be faid, perhaps, in fubterraneous caverns. To this we 
may anfwer, that he, who is but moderately acquainted with the 
ftru&ure of the earth, and the materials of which it is compofed, 
will be little inclined to allow of any great or extenfive caverns in 
it. But, though this fhould be admitted, how can it come to pafs 
that thefe caverns fhould not be filled with water ? If it is alleged, 
that the water is expelled, as the vapour is formed, why fhould 
not the vapour, as it is fuppofed to be the lighter, be expelled, 
rather than the water, by the fame paffages by which the water is 
to be expelled ? But let us fuppofe this difficulty alfo to be got 
over, and the water to be removed, and we fhall then have a gage 
for the denfity of the vapour ; for it muff be juft fufficient to make 
it capable of fuftaining a column of water, whofe height is equal 
to that of the furface of the fea above the bottom of the cavern, in 
which the vapour is fuppofed to be contained. Now, fince the 
mean weight of earth, ftones, &c. is not lefs than two and a half 
times the weight of water, this vapour muff be incrcafcd to two 
and half times its original elafticity, before it can, in any wife, 
raife the earth above it; and if we fuppofe it to be increafed to five 
times its original elafticity, it will then be no more than twice able 
to do fo ; in which cafe, fo much vapour only can be difeharged 
from the cavern, to produce an earthquake, as is equal to the 
content 
