328 Conjectures concerning the Cause, and Observations 
ter part will be driven forwards with greater velocity, but the 
foremost will travel slower, on account of its travelling under a 
thinner set of strata* ; and, besides this, the load being much less, 
it will greatly expand itself. From all these causes taken to- 
gether, the wave at the surface of the earth, occasioned by the 
passing of the vapour under it, will not only be much higher, but 
also much shorter, and, consequently, the sides of it, on both 
these accounts, will be much more inclined to the horizon: and, 
moreover, because the progress of the wave will be slower, it will 
give more time to any waters situated on one side of it, to flow 
one way; and on this account also, the apparent agitation of 
them will be increased. 
Section II.—73. We are told, that, in the Lisbon earth- 
quake of 1755, ‘‘the bar [at the mouth of the Tagus} was seen 
dry from shore to shore; then suddenly the sea, like a mountain, 
came rolling in; and about Bellem castle, the water rose fifty 
feet almost in an instant; and, had it not been for the great bay 
opposite to the city, which received and spread the great flux, 
the low part of it must have been under water +t.” The same 
phenomena were observed to accompany the same earthquake at 
the island of Madeira; where we are told, that, at the city of 
Funchal, ** the sea, which was quite calm, was observed to re- 
tire suddenly some paces; then rising with a great swell, with- 
out the least noise, and as suddenly advancing, it overflowed the 
shore, and entered the city. It rose full fifteen feet perpendi- 
cular above high-water mark, although the tide, which ebbs and 
flows there seven feet, was then at half ebb. In the northern 
part of the island, the inundation was more violent, the sea re- 
tiring there above one hundred paces at first, and suddenly re- 
turning, overflowed the shore, forcing open doors, breaking down 
the walls of several magazines and storehouses, and carrying away 
in its recess, a considerable quantity of grain, and some hundred 
pipes of wine {.” 
74. Both these appearances (which have been observed to at- 
tend several other earthquakes, as well as this) seem to admit of 
an easy solution, supposing the cause of them to lie under the 
bed of the ocean; for, in the further progress of the communi- 
cation between the fire and water, the vapour, that is gradually 
raised at first, will at last begin to raise the roof over the fire, 
which being supported by so light a vapour, there will now be 
no want of fluidity in the matter it rests upon, and the difference 
of specific gravity between the two, instead of being small, will be 
* See art. 63, the note. 
+ See Hist. and Philos. of Earthq. p. 316. 
} See Philos. Trans. vol. xlix. p. 432, &c. or Hist. and Philos. of Earthq. 
p. 329, 
very 
— 
