Founders of Seismology.—I. John Michell. 1038 
He then proceeds to show how his theory will account for the 
various phenomena described above. The return of earthquakes to 
the same place is to be expected, for, as in voleanoes, the sub- 
terraneous fires may be supposed to subsist for many ages. More- 
over, as volcanoes frequently rage for a time and then are quiet for 
a number of years, so we see earthquakes frequently repeated for 
some small time and then ceasing for a long period, except perhaps 
for the occurrence now and then of some slight shock. The great 
frequency of earthquakes in the neighbourhood of burning mountains 
is a strong argument of their proceeding from a cause of the same 
kind. 
The two forms of motion which Michell so clearly discerned in 
earthquakes are consequences, in his view, of the varying amount 
of the vapour generated. A small quantity of vapour almost 
instantly generated would produce the vibratory motion; a very 
large quantity would result in a wave-like motion. 
The first effect of the vapour, he says, would be to form a cavity 
between the melted matter and superincumbent earth by the 
compression of the materials immediately over the cavity, and “‘ 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 beyond their natural bounds, a dilatation will succeed 
to the compression ; and these two following each other alternately 
for some time a vibratory motion will be produced at the surface 
of the earth. If these alternate dilations and compressions should 
succeed one another at. very short intervals, they would excite a 
like motion in the air, and thereby occasion a considerable noise. 
The noise,” he adds, “‘ that is usually observed to precede or accom- 
pany earthquakes 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.” 
Michell illustrates the propagation of the wave-like motion by 
describing an experiment which has led to some misconception 
of his views. ‘‘ Suppose a large cloth or carpet (spread upon 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 until it escapes at the opposite edge, 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 an 
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 turn, till it either finds some vent or is 
again condensed by the cold into water, and by that means prevented 
from proceeding any further.”’ 
