i6 A Yorkshire Rector of the Eighteenth Century. 



Michell conceived that if a body of water was let down 

 suddenly on one of these tracts of internal combustion, aqueous 

 vapour would be generated, the volume and elastic force of 

 which would suffice to give rise to an earthquake. In the 

 case of volcanoes, the vapour has been able to break its way 

 out to the surface at some weaker parts of the earth's crust, 

 and to project huge blocks of stone to great distances. ' If, 

 then,' he asks, ' when the vapours find a vent, they are capable 

 of shaking the country to a distance of ten or twenty miles 

 round, what may we not expect of them when they are con- 

 fined ? ' He distinguished two kinds of motion in an earthquake, 

 one tremulous over the centre of disturbance and for some 

 some distance around, the other undulatory in a succession of 

 waves that extend much further than the tremulous movement. 

 '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 very large quantity (whether 

 it be generated almost instantly, or in any small portion of 

 time) will produce a wave-like motion.' 



In illustration of what he conceived to be the manner in 

 which the wave-like motion is propagated, he supposed a 

 large carpet to be spread upon a floor, and after being raised 

 at one edge, to be 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.' He believed that as the air 

 travels along under the carpet which it uplifts in its progress, 

 so ' 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 going any farther.' (Art. 58). 



In volcanic districts, the sheets of water which often 

 accumulate in extinct craters, may occasionally be suddenly 

 precipitated into the region of the molten magma below, 

 with the consequent generation of much aqueous vapour. It 

 is conceivable that this vapour may force its way for some 

 distance between the planes of easily separable strata. The 

 minor earthquakes in volcanic regions may sometimes be due 

 to this cause, which, however, cannot te accepted as the cause 

 of earthquakes in general. But though Michell's 'conjecture' is 

 set aside, we should not fail to note that in making it he has 



NaCiiralisf , 



