[ 6l 4 ] 
way, and produce a great wave immediately fucceed- 
ing the previous retreat *. 
Sect. III. 
7 6. That great quantity of water, which we have 
iuppofed to be let out upon fubterraneous fires, and,, 
by that means, to produce earthquakes, will fupply 
us with a reafon, why they obferve a fort of periodi- 
cal return. This water muff extinguifh a great por- 
tion of the burning matter, in confequence of which, 
it will be contradted within much narrower bounds ; 
and though the etfedls before defcribed could not 
take place at firft, but by the great extenfion of the 
heated matter, yet, after they have once taken place,. 
* Tt may, perhaps, be objected, that thefe phaenomena may a&^ 
eafily be occafioned by a vapour generated under the dry land, 
which, by firft raifing the earth upon the fea-fhore, would make 
the waters retreat ; and that the return of them again, upon its 
fubfiding into its place, might caufe the fubfequent wave. That 
this may be the cafe, in feme inftances, is not impoffible, but, I 
believe, upon examining the particular circumftances, it will ge- 
nerally be found to.Jbe otherwise ; and there cannot be any doubt 
about it, in the cafe of the Lifbon earthquake; for the retreat w r as 
obferved to precede the wave, not only on the coaft of Portugal, 
but alfo at the ifland of Madeira, and feveral other places : now, 
if the retreat had been caufed by the raifing of the earth on the 
coaft of Portugal, the motion of the waters occafioned by this 
means, when propagated to Madeira, muft have produced a wave 
there previous to the retreat, contrary to what happened ; nor 
could the motion of the waters at Madeira be caufed by the earth- 
quake at that place, becaufe it did not happen till above two hours 
after; whence it is manifeft, that it muft have been owing to the 
continuation of a motion propagated from the place, where the 
earthquake exerted its firft efforts. And we may obferve, in gene- 
ral, that this muft always be the cafe, whenever the retreat does 
not happen till fome confiderable time after the earthquake. 
