106 Founders of Seismology.—I. John Michell. 
In locating the origin of the Lisbon earthquake Michell relies on 
other evidence rather than on the methods which he describes so 
clearly. The great sea-wave which followed the earthquake 
indicates that the origin was submarine, and it could not be far from 
Lisbon, as the wave arrived there so soon after the earthquake and 
was of such great height. It must, he thinks, lie somewhere between 
the latitudes of Lisbon and Oporto, though probably somewhat 
nearer to the former (about a degree of a great circle, he afterwards 
assumes, from Lisbon and a degree and a half from Oporto), and at 
a distance of 19 or 15 leagues from the coast. Such a position agrees 
well with the few observations which he possessed on the direction 
and time of the earthquake. But, curiously enough, he notices that 
“ the times which the wave took up in travelling are not in the same 
proportion with the distances of the respective places from the 
supposed source of the motion”. He does not, however, regard this 
as an objection to the point assumed, but thinks—and here he fore- 
saw a later discovery—that “‘the true reason of this disproportion 
seems to be the difference in the depth of the water ; for in every 
instance in the above table the time will be found to be 
proportionately shorter or longer as the water through which the 
wave passed was deeper or shallower ”’. : 
The methods which Michell suggests for inquiring “ into the 
depth at which the cause lies that occasions any particular earth- 
quake ’’ depend on his theory of the origin of earthquakes, and need 
not be here described. For the Lisbon earthquake the observations 
required in order to apply the methods did not exist. But “if”, 
he says, “I might be allowed to form a random guess about it I 
should suppose (upon a comparison of all circumstances) that it 
could not be much less than a mile or mile and a half, and I think 
it is probable it did not exceed three miles.” 
Conclusion.—With our fuller knowledge it would be an easy 
matter to criticize Michell’s theory of the cause of earthquakes. 
I do not propose to enter now on such a task, my object being rather 
to give a brief summary of his contributions, which seem to me to 
possess a permanent value. 
In the first place must be mentioned his distinction between 
the phenomena which are essential and those which are not essential 
to his theory. In separating the vibratory motion from the wave- 
like motion, Michell was in advance of his time He was one of the 
first, if not the very first, to assign the vibratory motion in earth- 
quakes te the propagation of elastic waves in the earth’s crust. 
In the second place he rendered animportant service in his attempt 
40 give a consistent theory of the origin of earthquakes, even if the 
theory, as we can now see, rests on an erroneous conception of 
voleanic action, and if it cannot explain all the phenomena that have 
since been discovered. His suggestions as to the origin of earth- 
quake sounds and of seismic sea-waves would seem, in part, at any 
rate, to be correct. 
