[ 6i 3 ] 
gradually raifed at firft, will at lad; begin to raife the 
roof over the fire, which, being fupported by fo light 
a vapour, there will now be no want of fluidity in the 
matter it reds upon, and the difference of fpecific 
gravity between the two, indead of being fmall, will 
be very great : hence, if any part of the roof gives 
way, it mud immediately fail in, the vapour readily 
rifing, and taking its place; and a beginning being 
once made, a communication will be opened with 
numberlefs clefts and fiflures, that mud occafion the 
falling in of vad quantities of matter, which, as foon 
as the vapour can pafs round them, will want their 
fupport ; then will follow the great * effects already 
delcribed. 
75. Now, whild the roof is railing, the waters of 
the ocean, lying over it, mud retreat, and flow from 
thence every way ; this, however, being brought 
about flowly, they will have time to retreat fo gently, 
as to occafion no great diflurbance : but as foon as fome 
part of the roof falls in, the cold water contained in 
the fiflures of it, mixing with the fleam, will imme- 
diately produce a vacuum, in the fame manner a’S 
the water injedted into the cylinder of a fleam engine, 
and the earth fubfiding, and leaving a hollow place 
above, the waters will flow every way towards it, 
and caufe a retreat of the fea on all the fhores round 
about : then prefently, the waters being again con- 
verted by the contadt of the fire into vapour, together 
with all the additional quantity, which has now an 
open communication with it, the earth will be raifed, 
and the waters over it will be made to flow every 
* See art. 56 to 60 inclufive. 
4K 2 
wav,. 
0 9 
