4(iO THILOSOPHICAL TRANSACTIONS. [anNO l/OO. 



tion in tl'.e air, and thereby occasion a considerable noise. The noise that is 

 usually observed to precede or accompany earthquakes, is probably owing partly 

 to this cause, and partly to the grating of the parts of the earth together, oc- 

 casioned by that wave-like motion before mentioned. After the water, that first 

 came in contact with the fire, has formed a cavity, all the rest of the water con- 

 tained in the fissures, iinmediately communicating with the hollow left by the 

 part that fell in, must run out upon the fire, the steam taking its place. Hence 

 may be generated a vast quantity of vapour, the effects of which shall be con- 

 sidered presently. This steam will continue to be generated, supposing the fire 

 to be sufficiently great, till the fissures before mentioned are evacuated, or till 

 the water begins to flow very slowly; when the steam already formed will be re- 

 moved by the elasticity of the earth, which will again subside, and pressing on 

 the surface 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 communica- 

 tion between the fire and the water will be prevented, excepting at these clefts, 

 where the water, dripping slowly on the melted matter, will gradually form a 

 crust on it, that will soon stop all further communication in these places like- 

 wise , and the fissures, that had been before evacuated, will be again gradually 

 replenished by the oozing of the water between the strata. 



As a small quantity of vapour almost instantly generated at some considerable 

 depth below the surface of the earth, will produce a vibratory motion, so a large 

 quantity will produce a wave-like motion. The manner in which this wave-like 

 motion will be propagated, may in some measure be represented by the follow- 

 ing experiment. Suppose a large cloth or carpet spread on a floor, to be raised 

 at one edge, and then suddenly brought down again to the floor, the air under 

 it being by this means propelled, will pass along till it escapes at the opposite 

 side, raising the cloth in a wave all the way as it goes. In like manner, a large 

 quantity of vapour may be conceived to raise the earth in a wave, as it passes 

 along between the strata, which it may easily separate in a horizontal direction, 

 there being little or no cohesion between one stratum and another. The part of 

 the earth that is first raised, being bent from its natural form, will endeavour to 

 restore itself by its elasticity, and the parts next to it beginning to have their 

 weight supported by the vapour, which will insinuate itself under them, will be 

 raised in their tuni, till it either finds some vent, or is again condensed by the 

 cold into water, and by that means prevented from proceeding any farther. 



If a large quantity of vapour should continue to be generated for some time, 

 several waves might be produced by it; and this would be in some measure the 

 case if the quantity at first generated was exceedingly great, though the whole of 

 it was generated in less time than while the motion was propagated through the 

 distance between two waves. These waves must rise the higher, the nearer they 



