upon the Phenomena, of Earthquakes. 327 
about an hour and half after the former; which latter agrees with 
the times when the agitations of the waters were observed in 
England, if we allow only a proper interval for the motion to be 
propagated so far northward, proportionably to the time it took 
up in travelling from its original source near Lisbon. 
71. These appearances seem to be connected with that men- 
tioned in the preceding article, and they may both, I think, be 
accounted for, by supposing a considerable quantity of vapour to 
be raised, whilst the partition before-mentioned was beginning 
to give way; during which time a partial communication be- 
tween the water and fire would be brought on, and that by de- 
grees only. Hence the vapour, not being produced at once but 
gradually, might creep silently between the strata*, towards that 
quarter where the superincumbent mass cf earth was lightest ; 
and, by this means, some places very near the source of the va- 
pour might be little, or not at all, affected by it, whilst others 
might be greatly affected, though they lay at a great distance 3 
and even those places, which lay immediately over the part where 
the vapour was passing, might not perceive any effect, on ae- 
count of the gentleness of the motion, occasioned by the small 
quantity of it. This might continue to be the case, till it came 
to some country where, the set of strata above being much 
thinner, the vapour would not only be-hurried forward, but col- 
jected also into a much narrower compass ; and therefore, raising 
the earth more, would produce more sensible effects; and this 
we ought chiefly to expect in the most mountainous countries, 
according to the idea before given of them +. 
72. To make this something clearer, let us suppose, in fig. 1. 
the vapour to be passing between the strata in the dotted line C, 
and to go forwards, till it arrives at A: whilst, then, it passes 
under the deeper parts at E, it will raise the earth over it but 
little, as well because it will be spread broader and thinner, as 
because it will be more compressed by the weight of the super- 
incumbent matter ; but as it arrives towards A, not only the lat- 
* Some appearances that have been observed in New England seem to 
confirin this, and make it probable, that a small quantity of vapour is often 
found to creep silently between the strata, before a general communication 
between the water and the fire gives rise to the greater and more sensible 
effects of earthquakes. See Philos. Trans.No. 462 ; or Martyn’s Abr.vol. viii. 
p- 693, where we are told, that, at Newbury, a little before any noise or 
shock was perceived, the bricks of an hearth were observed to rise, and, 
falling down again, to lean another way. In the same account, it is also 
said, that “‘ a few minutes before any shock came, many people could fore- 
tell it by an alteration in their stomachs :” an effect, which seems to be of 
the same kind with sea-sickness, and which always accompanies the wave- 
like motion of earthquakes, when it is so weak, as to be uncertainly distin- 
guishable. ft See art. 43. 
-X 4 ter 
