concerning the Phenomena of Earthquakes. 26% 
sponding strata on the other side. If, in some cases, this dif- 
ference in the level of the strata, on the different sides of the cleft, 
should be very considerable, it may have a great effect in pro- 
ducing some of the singularities of particular earthquakes*. 
Parr II.—51. In the former part of this essay, I have re- 
counted some of the principal appearances of earthquakes, as 
well as those particulars in the structure of the earth, upon which 
I suppose these appearances to depend. From what has been 
already said, I think it is sufficiently manifest, that, in some in- 
stances at least, earthquakes are actually produced by subter— 
raneous fires: it now, therefore, remains to be shown, how alk 
the appearances above-recited, as well as many other minuter 
circumstances attending earthquakes, may be accounted for from 
the same cause. 
SEcTION I.—52. The returns of earthquakes in the same places, 
either at small or large intervals of time, are very consistent 
with the cause assigned: subterraneous fires, from their analogy 
to voleanos, might reasonably be supposed to subsist for many 
ages, though we had not those instances already mentioned +, 
which put the matter out of doubt. And, as it frequently hap- 
pens, that volcanos rage for a time, and then are quiet again for 
a number of years; so we see earthquakes also frequently re- 
peated for some small time, and then ceasing again for a long 
term, excepting, perhaps, now and then some slight shock. 
And this analogy between earthquakes, and the effects of vol- 
canos, is so great, that I think it cannot but appear striking to 
any one, who will read the accounts of both, and compare them 
together. The raging of volcanos is not one continued and uni- 
form effect ; but an effect, that is repeated at unequal intervals, 
and with unequal degrees of force : thus, for instance, we shall 
have, perhaps, two or three blasts discharged from a volcano, 
succeeding one another at the interval of a few seconds only: 
sometimes the intervals are of a quarter of an hour, an hour, a 
day, or perhaps several days. And as these intervals are very un- 
equal, so is the violence of the blasts also: sometimes stones, &c. 
are thrown, by these blasts, to the distance of some miles; at 
other times, perhaps, not to the distance of a hundred yards. 
The same difference is observed in the intervals and violence of 
* Fig. 2. represents a section of the strata trapping down after the man- 
ner just described. The section is supposed to be made perpendicularly to 
the 5 aparcay and at right angles to the direction of the cleft: an instance 
of this kind, amongst the coal mines of Mendip in Somersetshire, is men-~ 
tioned in the Phil. Trans. See the account of it, together with a drawing, 
in No, 360, er Jones's Abr. vol. iy. part ii. p. 260. 
+ See art. 28 to $2 inclusive. : 
R 3 the 
