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muft run out upon the fire, the fleam taking its 
place. From hence may be generated a vaft quantity 
of vapour, the effedfs of which Shall be confidered 
prefently. This fleam will continue to be generated, 
fuppofing the fire to be fufficiently great, till the fif- 
fures before-mentioned are evacuated, or till the wa- 
ter begins to flow very flowly ; when the fleam al- 
ready formed will be removed by the elasticity of 
the earth, which will again fubfide, and, prefling 
upon the furface of the melted matter, will force it 
up a little way into all the clefts, by which the water 
might continue to flow out. By this means, all com- 
munication between the fire and the water will be 
prevented, excepting at thefe clefts, where the water, 
dripping flowly upon the melted matter, will gra- 
dually form a cruft upon it, that will foon ftop all 
farther communication in thefe places likewife; and 
the fiflures, that had been before evacuated, will be 
again gradually replenished by the oozing of the water 
between the ftrata. 
58. As a fmall quantity of vapour almoft inftantly 
generated at fome considerable depth below the fur- 
face of the earth, will produce a vibratory motion. 
So a very large quantity (whether it be generated al- 
moft inftantly, or in any fmall portion of time) will 
produce a wave-like motion^ The manner in which 
this wave-like motion will be propagated, may, in 
fome meafure, be reprefented by the following expe- 
riment. Suppofe a large cloth, or carpet, (Spread 
upon a floor) to be railed at one edge, and then 
Suddenly brought down again to the floor, the air 
under it, being by this means propelled, will pafs 
along, till it efcapes at the oppofite fide, railing the 
cloth 
