[ 5 6 9 3 
and all the other circumdances attending thefe mo- 
tions, fliould be owing to fome caufe redding in 
the air. 
f. Let us then, rejecting this hypotheds, fuppofe, 
that earthquakes have their origin under ground, and 
we need not go far in feareh of a caufe, whofe real 
exidence in nature we have certain evidence of, and 
which is capable of producing all the appearances of 
thefe extraordinary motions. The caufe I mean is 
fubterraneous fires. Thefe fires, if a large quantity 
of water fhould be let out upon them fuddenly, may 
produce a vapour, whofe quantity and elaftic force 
may be fully fufficient for that purpofe. The prin- 
cipal fadts, from which I would prove, that thefe 
fires are the real, caufe of earthquakes, are as follow. 
Section I. 
6. Firjl , The fame places are fubjedt to re- 
turns of earthquakes, not only at fmall inter- 
vals for fome time after any condderable one 
has happened, but alfo at greater intervals of 
fome ages. 
7. Both thefe fadls diffidently appear, from the 
accounts we have of earthquakes. The tremblings 
and (hocks of the earth at * Jamaica in 1 692, at 
* Sicily in 1693, and at * Lilbon in 1 7ff, were re- 
peated fometimes at larger, and fometimes at fmaller 
intervals, for feveral months. The fame thing has 
been obferved in all other very violent earthquakes. 
At “f- Lima, from the 28th Odtober 1746, to the 
* See the accounts of thefe in the Philof. Tranf. 
f. See Antonio d’Ulloa’s Voyage to Peru, part ii. book i. ch. 7. 
9 24th 
