[ 6o6 ] 
gradually filled k with water, as it is formed, the 
melted matter being prevented from filling it, by 
its want of fluidity, as well as on account of the 
other circum fiances, under which it is to fpread- 
itfelfj for the lentor and iluggifhnefs of this kind 
of matter is inch, that, when fomewhat cooled on 
the furface by the contad of the air only, it will not 
flow, perhaps, ten feet in a month, though in a very 
large body ; infiances of which we have in the lavas- 
of /Etna, Vefuvius, &c. It is not to be expeded 
taen, that it fhould fpread far, when it comes in con- 
nd with water at its edges, as fpon as it is formed, 
and when it is, perhaps, feveral months in acquiring 
a thicknefs of a few inches ; but it muff, by degrees, 
form a kind of wall between the fire and the open- 
ing into the annular fpace befpre described. This 
wall will gradually increafe in height, till it becomes 
too tall in proportion to its thicknefs, to bear any 
longer the prefiure of the melted matter ; which 
approach towards thofe which are fometimes found in nature : we 
may fuppofe then the ftratum B to he, perhaps, from ten or twenty 
to a hundred yards in thicknefs; the greateft height of the annular 
fpace next the fi re, to be from four or five to ten or fifteen 
feet, and its greateft extent, horizontally, from ten or twenty to 
fi!ty or fixty 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.J and the thicknefs of the ftiperincumbent matter at D, 
may be from a quarter or half a mile to two or three miles ; the 
number of the laminte a!fo, into which it is divided, may he many 
times more than thofe in the figure. As to the perpendicular 
fifiures, they muft be fo numerous, and fo finall, in proportion 
to the other parts, that I chofe rather to leave them, to he fupplied 
by the imagination of the reader, than attempt to exprefs them in a 
manner, that could give no adequate idea of them at all. 
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mufi 
