104 Founders of Sevsmology.—I. John Michell. 
Equally interesting, and less open to objection, is Michell’s 
explanation of the propagation of the wave-like motion to distances 
far greater than the vibratory motion. This motion, he states, 
“will be propagated through the solid parts of the earth, and, 
therefore, it will much sooner become too weak to be perceived than 
the wave-like motion, for this latter, being occasioned by the vapour 
insinuating itself between the strata, may be propagated to very 
great distances ; and even after it has ceased to be perceived by the 
senses it may still discover itself by the appearances before 
mentioned.” That is to say, im modern terms, the wave-like motion 
is propagated in two and the vibratory motion in three dimensions. 
All earthquakes derived from the same subterraneous fire must 
come to any place in the same direction. Those only which are 
derived from different fires will come from different points of the 
compass. Moreover, since, according to Michell, the velocity with 
which the vapour insinuates itself between the strata depends upon 
its depth below the surface, earthquakes must travel with the same - 
velocity if they proceed from fires at different depths. The 
occurrence of local earthquakes in other places Michell accounts for 
by supposing that the vapour coming from one fire may disturb the 
roof over some adjacent fires, and so occasion earthquakes by the 
falling in of some part of it. 
Michell also suggests explanations of two other phenomena 
besides those described above. These are the great sea-waves and 
“a sort of periodical return ’’ of earthquakes in the same region. 
The first movement of the sea at Lisbon was, as usual, a retreat of 
the water followed by the inrush of the great waves. During the 
elevation of the roof over the subterraneous fire the waters of the 
ocean lying over it must retreat, but so slowly and so gently as to 
occasion no great disturbance. But as soon as one part of the roof 
falls in “‘ the cold water contained in the fissure of it, mixing with the 
steam, will immediately produce a vacuum, in the same manner 
as the water injected into the cylinder of a steam-engine, and the 
earth subsiding, and leaving a hollow place above, the waters will 
flow every way towards it and cause a retreat of the sea on all the 
shores round about; then, presently, the waters being again con- 
verted by the contact of the fire into vapour . . . the earth will be 
raised, and the waters over it will be made to flow every way and 
produce a great wave immediately succeeding the previous retreat.” 
Lastly, the great quantity of water let out upon the sub- 
terraneous fire “must extinguish a great portion of the burning 
matter, in conseyuence of which it will be contracted within much 
narrower bounds’”’, and this will continue ‘till the roof is well 
settled and the surface of the melted matter sufficiently cooled, after 
which it may require a long time for the fire to heat it again so much 
as will be necessary to make it produce the former effects *’.t 
1 At the end of the section on stratification Michell gives the following 
paragraph: ‘‘ Besides the rising of the strata in a ridge, there is another very 
