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and if the fame * firata fhould be on fire in any places 
under fuch countries, as well as on the tops of the hills, 
all vapours, of whatfoever kind, raifed from thefe 
fires, muft be pent up, unlefs fo far as they can open 
themfelves a paflage between the firata; whereas the 
vapours raifed from volcanos find a vent, and are dis- 
charged in blafis from the mouths of them. Now, 
if, when they find fuch a vent, they are yet capable 
of fiiaking the country to the diftance of ten or twenty 
miles round, what may we not expedt from them, 
when they are confined ? We may form fome idea 
of the force and quantity of thefe vapours from their 
efiedts: it is no uncommon thing to fee them throw 
up, at once, fuch clouds of fand, allies, and pumice 
fiones, as are capable of darkening the whole air, 
and covering the neighbouring country with a fiiower 
of duft, &c, to fome miles difiance: great fiones 
alfo, of fome tons weight, are often thrown to the 
difiance of two or three miles by thefe explofions : 
and Monf. Bouguer tells us, that he met with fiones 
* It may be afked, perhaps, why a ftratum liable to take fire 
in fome places, fhould not take fire throughout the whole extent 
of it ? In anfwer to this, it may be faid, that the fame ftratum 
may differ a little in the richnefs of its combuflible principles in 
different places ; or, perhaps, the frequency of the fiflures, either 
in the combuflible ftratum itfelf, or the ftratum next to it, may let 
in fo much water, as to prevent its taking fire, excepting in a few 
places ; but, if this once happens, the fire will not eafily be put out 
again, but it will fpread itfelf, notwithftanding the fiflures that lie in 
its way, though they are filled with water ; for the matter on fire 
will be, in fome degree at leaft, in a fluid ftate; and, for this reafon, 
it muft neccflarily expel the water from the fiflures, both on ac- 
count of the extenfion of its own dimenfions by the heat, and of 
the weight of the fuperincumbent earth, which, prefling it, will 
make it fpread laterally. 
ill 
