‘Founders of Seismology.—TI. John Michell. 105 
The Position of the Epicentre.—In the last section of his memoir, 
Michell shows that “‘ if we would inquire into the place of the origin 
of any particular earthquake, we have the following grounds to go 
upon :— 
(i) ‘‘ The different directions in which it arrives at several distant 
places ; if lines be drawn in these directions the place of their common 
intersection must be nearly the place sought ; but this is liable to 
great difficulties, for there must necessarily be great uncertainty 
in observations, which cannot, at best, be made with any great 
precision, and which are generally made by minds too little at ease 
to be nice observers of what passes ; moreover, the directions them- 
selves may be somewhat varied by the inequalities in the weight 
of the superincumbent matter, under which the vapour passes, as 
well as by other causes. 
(ii) ““ We may form some judgment concerning the place of the 
origin of a particular earthquake from the time of its arrival at 
different places, but this also is liable to great difficulties. In both 
these methods, however, we may come to a much greater degree 
of exactness by taking a medium amongst a variety of accounts 
as they are related by different observers. But, 
(iii) ““ We may come to the greatest degree of exactness in those 
cases where earthquakes have their source from under the ocean ; 
for in these instances the proportional distance of places from that 
source may be very nearly ascertained by the interval between the 
earthquake and the succeeding wave, and this is the more to be 
depended on as people are much less likely to be mistaken in 
determining the time between two events which follow one another 
at a small interval than in observing the precise time of the 
happening of some single event.”’ 
The first method is that which Mallet applied with such skill 
in his study of the Neapolitan earthquake of 1857, though he 
depended on more permanent evidences of the direction than the 
impressions of observers. The third method is, of course, vitiated 
by the assumption that the velocity of sea-waves is independent of 
the depth of the ocean, but the greater accuracy attainable by 
measuring intervals rather than absolute times lies at the root of 
the method depending on the duration of the preliminary tremor 
which is now so widely used. 
remarkable appearance in the structure of the earth, though a very common 
one; and this is what is usually called by miners the trapping down of the 
strata; that is, the whole set of strata on one side such a cleft are sunk down 
below the level of the corresponding strata on the other side. If, in some 
_cases, this difference in the level of the strata on the different sides of the 
cleft should be very considerable, it may have a great effect in producing some 
of the singularities of particular earthquakes.’’ One cannot help regretting 
the brevity of Michell’s statement on the relations between earthquakes and 
faults. I can only suggest that, according to his view, the vapour, in 
travelling outwards by parting adjacent strata, would be suddenly arrested at 
the fault. 
