upon the Phenomena, of Earthquakes. 329 
very great: hence, if any part of the roof gives way, it must im- 
mediately fall in, the vapour readily rising, and taking its place; 
and a beginning being once made, a communication will be 
opened with numberless clefts and fissures, that must occasion the 
falling in of vast quantities of matter, which, as soon as the va- 
pour can pass round them, will want their support ; then will fol- 
low the great effects* already described. 
75. Now, whilst the roof is raising, the waters of the ocean, 
lying over it, must retreat, and flow from thence every way ; this 
however, being brought about slowly, they will have time to re- 
treat so gently, as to occasion no great disturbance: but as soon 
as some part of the roof falls in, the cold water contained in the 
fissures of it, mixing with the steam, will immediately produce a va- 
cuum, in the same manner as the water injected into the cylinder of 
a steam-engine,and the earthsubsiding, and leaving a hollow place 
above, the waters will flow every way towards it, and cause a re- 
treat of the sea on all the shores round about: then presently, the 
waters being again converted by the contact of the fire into va- 
pour, together with‘all the additional quantity, which has now 
an open communication with it, the earth will be raised, and the 
waters over it will be made to flow ever y way, and produce a great 
wave immediately succeeding the previous retreat T. 
Section II].—76. That great quantity of water, which we 
have supposed to be let out upon subterraneous fires, and, by that 
means, to produce earthquakes, will supply us with a reason, 
why they observe a sort of periodical return. ‘This water must 
extinguish a great portion of the burning matter, in consequence 
of which, it will be contracted within much narrower bounds ; 
and though the effects before described could not take place at 
* See art. 56 to 60 inclusive. 
+ It may, perhaps, be objected, that these phenomena may as easily be 
occasioned by a vapour generated under the dry land, which, by first raising 
the earth upon the sea-shore, would make the waters retreat; and that the 
return of them again, uponits subsiding into its place, might cause the sub- 
sequent wave. That this may be the case, in some instances, is not im- 
possible, but, I believe, upon examining the particular circumstances, it 
will gener ally be found to be otherwise; and there cannot be any doubt 
about it, in the case of the Lisbon earthquake ; for the retreat was observed 
to precede the wave, not only on the coast of Portugal, but also at the island 
of Madeira, and several other places: now, ifthe retreat had been caused 
by the raising of the earth on the coast of Portugal, the motion of the waters 
occasioned by this means, when propagated to Madeira, must have produced 
a wave there previous to the retreat, contrary to what happened ; nor could 
the motion of the waters at Madeira be caused by the earthquake at that 
place, because it did not happen till above two hours after; whence it is 
manifest, that it must have been owing to the continuation of a motion pro- 
pagated from the place, where the earthquake exerted its first efforts. And 
we may observe, in general, that this must always be the case, whenever the 
retreat does not happen till some considerable time after the earthquake. 
first, 
