upon.the Phenomena, of Earthquakes. 267 
diately sink in the melted. matter of the fire below: hence all the 
water contained in the fissures and cavities of the part falling in, 
would come in contact with the fire, and be almost instantly 
raised into vapour. From the first effort of this vapour, a cavity 
would be formed (between the melted matter and superincum- 
bent earth) filled with vapour only, before any motion would be 
perceived at the surface of the earth: this must necessarily hap- 
- pen, on account of the compressibility* of all kinds of earth, stones, 
&c. but as the compression of the materials immediately over 
the cavity, would be more than sufficient to make them bear the 
weight of the superincumbent matter, this compression must be 
propagated on account of the elasticity of the earth, in the same 
manner as a pulse is propagated through the air; and again the 
materials immediately over the cavity, restoring themselves be- 
yond their natural bounds, a dilatation will succeed to the com- 
pression ; and these two following each other alternately, for 
some time, a vibratory motion will be produced at the surface of 
theearth, If these alternate dilatations and compressions should 
succeed one another at very small intervals, they would excite a 
like motion in the air, and thereby occasion a considerable noise. 
The noise that is usually observed to precede or accompany earth- 
quakes, is probably owing partly to this cause, and partly to the 
grating of the parts of the earth together, occasioned by that 
wave-like motion before mentioned. 
57. After the water, that first came in contact with the fire, 
has formed a cavity, all the rest of the water contained in the 
fissures, 
* The compressibility and elasticity of the earth, are qualities which don’t 
show themselves in any great degree in common instances, and therefore 
are not commonly attended to. On this account it is, that few peuple are 
aware of the great extent of them, or the effects that may arise from them, 
where exceeding large quantities of matter are concerned, and where the 
compressive force is immensely great.. The compressib'lity and elasticity 
of the earth may be collected, in some measure, from the vibration of the 
walls of houses, occasioned by the passing of carriages in the streets next 
to them. Another instance to the same purpose, may be taken from the 
vibrations of steeples, occasioned by the ringing of bells, or by gusts of 
wind: not only spires are moved very considerably by this means, but 
even strong towers will, sometimes, be made tg vibrate several inches, with- 
out any disjointing of the mortar, or rubbing of the stones against one an- 
other. Now, itis manifest, that this could not happen, without a consi- 
derable degree of compressibility and elasticity in the materials, of which 
they are composed: and if such small things as the weight of stecples, and 
the motion of bells in them, or a gust of wind, are capable of producing 
such effects, what may we not expect from the weight of great depths of 
earth? There are some circumstances, which seem tv make it not alto- 
gether improbable, that the fourm and internal structure of the earth de- 
pend, in a great measure, upon the compressibility aud elasticity of it. 
‘There are several things that seem to argue a considerably greater density 
in the internal, thag the external part of the carth; and why say not this 
greater 
