[ 6i 5 ] 
they may well continue to do fo for fome time j for 
the great difturbance in the firft inflance, by the 
falling in of a great part of the roof, rauft render the 
frequent communication between the fire and water 
not only very eafy, but almofi: unavoidable : and this 
will continue to be fo, till the roof is well fettled, 
and the furface of the melted matter fufficiently 
cooled, after which, it may require a long time for 
the fire to heat it again fo much, as will be neceflary 
to make it produce the former effects. Now, as the 
matter has been more or lefs cooled, or as the com- 
buftible materials are with more or lefs difficulty fet 
on fire again, as well as on account of other circum- 
ftances, the returns of thefe effects will be later or 
earlier i but though they will not, for this reafon, 
obferve any exadt period, yet they will generally fall 
within fome fort of limits, till either the matter that 
occafions them is confumed, (which, probably, will 
feldom happen in lefs than many ages) or till the 
fires open themfelves a paffage, and become vol- 
canos. 
Sect. IV. 
77. I have already intimated, that the moil: exten- 
five earthquakes frequently take their rife from the 
fea. According to the description of the * ftrudture 
of the earth before given, any combuftible ftratum 
muff lie at greater depths in places under the ocean, 
than elfewhere; hence far more extenfive fires may 
fubfifi: there, than where the quantity of matter over 
them is lefs ; for any vapour raifed from fuch fires. 
* S 
having 
ee art. 43. 
