[ 6 °4 ] 
pafiss at fame diftance under it, or between the 
iatuni, in which the fire lies, and that next above- 
or below it. 
PART III. 
Sect. I. 
6f. TN the former part of this trad, I fuppofed 3. 
part of the roof over fome fubterraneous fire 
to fall in . this is an event that cannot happen merely 
accidentally ; for fo long as the roof reds on the mat- 
ter on fire, no part of it can fall in, unlefs the matter 
elow could rife and take its place : now, it is very 
difficult to conceive how this ffiould happen, unlefs it 
was to rife by fome larger padages than the ordinary 
finures of the earth, which feem much too narrow 
foi that purpofej for, befides that the melted matter 
cannot be fuppofed to have any very great degree of 
fluidity, it mud necefiarily have a hard crud formed 1 
upon it, at all the fifiures, by the long continued con- 
tad of the water contained in them : thefe impede 
ments feem too great to be overcome by the did'erence 
of the fpecific gravities of the part that is to fall in, 
and the melted matter, which is the only caufe that 
can tend to make it defcend j the manner therefore, 
in which, I fuppofe, this event may be brought 
about, is as follows : 
66 . The matter of which any fubterraneous fire is 
compofed, mud be greatly * extended beyond its ori- 
ginal 
. j "I’ b0di f S we are ac q u ainted with are liable to be ex- 
Cl. c y ieat, there can be no doubt of its being fo in this cafe 
likewife 
