Founders of Seismology.—tI. John Michell. 101 
(iii) The motion of the earth in earthquakes is partly tremulous 
and partly propagated by waves, which succeed one another sonie- 
timesat larger, sometimes at smaller, distances; and this latter motion 
is generally propagated much farther than the other. The former 
part of this statement, Michell considers, ‘‘ wants no confirmation.” 
Indeed, his explanation of the manner in which the vibrations are 
propagated through the solid crust is quite as clear as that given by 
Mallet nearly a century later. In illustration of the latter, he quotes 
accounts of what are now called “ visible waves”’ , such as that of an 
observer of the Jamaica earthquake of 1688, who ‘saw the eround 
rise like the sea in a wave as the earthquake passed along, and ”’ 
who ‘‘ could distinguish the effects of it to some miles distance by 
the motion of the ‘tops of the trees on the hills”. In the Lisbon 
earthquake ‘ this wave-like motion was propagated to far greater 
distances than the other tremulous one, being perceived by the 
motion of waters and the hanging branches in churches through all 
Germany . . . in Denmark, Sele, Norway, and all over the 
British Isles ”’ 
(iv) It is observed in places which are subject to frequent earth- 
quakes that they generally come to one and the same place from the 
same point of the compass. Also the velocity with which they 
proceed (as far as one can collect it from the accounts of them) 1s 
the same; but the velocity of the earthquakes of diflerent countries 
is very different. Michell’s illustration of the latter statement, 
though not quite accurate, is interesting. The velocity with which 
the Lisbon earthquake and its after-shocks were propagated “ was 
the same, being at least equal to that of sound; for all followed 
immediately after the noise that preceded them, or rather the noise 
and the earthquake came together”; that of the New England 
earthquakes was less, as 1s shown ay the longer interval between 
the preceding noise and the shock. 
(v) The oreat Lisbon earthquake was succeeded by several local 
ones (in Switzerland and elsewhere), the extent of which was 
much less. 
The Cause of the Earthquakes. —Long before Michell’s time, it was 
“the general opinion of philosophers that earthquakes owe their 
origin to some sudden explosion in the internal parts of the earth ”’. 
Michell traces earthquakes to the primary cause to which they also 
appeal, namely, subterraneous fires. He differs from his predecessors 
chiefly in his endeavour to support this theory by facts and to trace 
out the effects of such explosions. ‘‘ These fires,” he says, “if a 
1 From records of the time of the great earthquake at Lisbon and more 
distant places, Michell estimates the velocity at more than 20 miles a minute. 
This is the earliest estimate of the kind known to me. 
2 The following sentence is worth quoting from The History and Philosophy 
of Earthquakes (p. 26), as it refers to probably the earliest instance of a 
seismic experiment: ‘* Honoratus Faber illustrates this doctrine by a variety 
of artificial earthquakes, as he calls them, confining gunpowder (a mixture of 
nitre, sulphur, and charcoal) in pits, and setting fire to it by a train.’’ 
