ba A 
upon the Phenomena, of Earthquakes. 325 
what cooled on the surface by the contact of the air only, it 
will not flow, perhaps, ten feet in a month, though in a very large 
body; instances of which we have in the lavas of Etna, Vesuvius, 
&e. It is not to be expected then, that it should spread far, 
when it comes in contact with water at its edges, as soon as it is 
‘formed, and when it is, perhaps, several months in acquiring a 
thickness of a few inches; but it must, by degrees, form a kind 
of wall between the fire and the opening into the annular space 
before described. This wall will gradually increase in height, till 
it becomes too tall in proportion to its thickness, to bear any 
longer the pressure of the melted matter; which must necessarily 
happen at last, because the thickness of it will not exceed a cer- 
tain limit *. 
67. Besides the giving way of this wall, the fire may under- 
mine the space containing the water, and, by that means, open 
a communication between them. Let us suppose one of these 
come to pass, and the time arrived when the partition begins to 
yield. If then the water had any way to escape readily, the 
breach would be made, and the melted matter would burst forth 
immediately, and flow out in large quantities at once amongst 
it; but as this is not the case, and it can only escape by oozing 
slowly between the strata, and through the fissures, the way that 
it came, the breach will be made gradually, from whence we may 
account for some appearances that have preceded great earth- 
quakes. 
towards those which are sometimes found in nature: we may suppose then 
the stratum B to be, perhaps, from ten or twenty to a hundred yards in 
thickness; the greatest height of the annular space C, next the fire, to be 
from four or five to ten or fifteen feet, and its greatest extent, horizontally, 
from ten or twenty to fifty or sixty feet ; the horizontal extent of the fire at 
A, may be from half a mile to ten or twenty miles ; [see art. 29, and the note 
to art. 53.] and the thickness of the superincumbent matter at D, may be 
from a quarter or half a mile to two or three miles ; the number of the la- 
mine also, into which it is divided, may be many times more than those in 
the figure. As to the perpendicular fissures, they must be so numerous, 
and so small, in proportion to the other parts, that I chose rather to leave 
them, to be supplied by the imagination of the reader, than attempt to ex- 
press them in a manner, that could give no adequate idea of them at all. 
* This limit will depend upon the thickness of the matter necessary to 
prevent so quick a communication of the heat or cold through it, as that the 
water should be able to diminish the heat of the fire considerably. The 
thickness requisite to do this, is very different in different kinds of bodies. 
Metals of all kinds transmit heat and cold extremely readily; but bricks 
and vitrified substances (with which last we may class the matter under our 
present consideration) transmit them very slowly: the walls of the hottest 
of our furnaces, when built of bricks, and eighteen inches thick, will not 
transmit more heat than a living animal can bear without injury, though the 
fires are continued in them for ever so long a tima; probably, therefore, 
if we allow two feet for the thickness of the matter, cooled and rendered 
hard by the contact of the water, we shall not underdo it. 
XO 68. We 
