[ 597 ] 
motions will be brought about, let us fuppofe the 
roof over fome fubterraneous fire to fall in. If this 
fhould be the cafe, the earth, iiones, &c. of which 
it was compofed, would immediately fink in the 
melted matter of the fire below : hence all the water 
contained in the fifiures and cavities of the part fall- 
ing in, would come in contad: with the fire, and be 
almofl inftantly railed into vapour. From the firft 
effort of this vapour, a cavity would be formed (be- 
tween the melted matter and fuperincumbent earth) 
filled with vapour only, before any motion would be 
perceived at the furface of the earth : this mull ne- 
celfarily happen, on account of the * compreffibility 
content of the cavern : and what muft the fize of that cavern 
be, which could contain vapour enough to produce the earthquake 
of the i ft of November 1755, in which an extent of earth of near 
three thoufand miles diameter was conliderably moved ? or how can 
we fuppofe, that the roof of fuch a cavern, when fo violently 
fhaken, fhould avoid falling in? efpecially, as it is hardly to be 
fuppofed, that any inflamed vapour whatfoever fhould be able to 
move the earth over thefe caverns, if they lay at any great depth, 
fince the weight of lefs than three miles depth of earth is capable 
of retaining the inflamed vapour of gunpowder within the original 
dimenfions of the gunpowder itfelf ; and common air, comprefled 
by the fame weight (fuppofing the known law of its compreffion to 
hold fo far), would be of greater denfity than water. 
We may afk ftill farther, whence fuch vaft quantities of vapour 
fhould be formed, or what fources they muft be, which would 
not be exhaufted (if they were not again replenifhed) by a very few 
repetitions of fuch immenfe difeharges. 
* The compreffibility and elafticity of the earth, are qualities 
which don’t fhow themfelves in any great degree in common in- 
ftances, and therefore are not commonly attended to. On this 
account it is, that few people are aware of the great extent of 
, them, or the effects that may arife from them, where exceeding 
large quantities of matter are concerned, and where the compref- 
4 H 2 five 
