Observations upon the Phenomena of Earthquakes. 323 
The angle under which the shadow appears is taken in the 
same manner; and the moon’s semidiameter ;: this measure : : 
semidiameter in miles : the length of the shadow. 
As the result deduced from the above operation will always be 
the depth of that part of the cavity on which the extremity of the 
shadow falls, the difficulty of ascertaining when the shadow falls 
on the bottom may be objected to it :—this, however, may always 
be obviated by continuing the observations until their result be- 
comes a maximum, which will evidently be the depth of the 
deepest part ; or in those cavities in which prominences or bright 
spots appear, it may be more easily done by observing when one 
of these prominences (which are no doubt situated at the bot- 
toms of their respective cavities) is in the line which forms the 
boundary of the shadow. 
By repeated observations some idea may be formed of the in- 
terior shapes of these cavities. If from afew continued ‘mea- 
surements the same depths are deduced, it may be concluded that 
the bottom is a plane surface: if they are found gradually to in- 
crease and afterwards decrease in the same proportion, that part 
of the interior surface will be shown either to be formed by the 
inclination of two planes meeting at the bottom, or to be a curve : 
to which of these elasses it belongs may be ascertained from the 
nature of this increase. Any considerable irregularities in the 
bottom or sides will be marked by corresponding diminutions in 
the depth of the cavity, or length of theshadow. Whether these 
speculations are carried further than is sanctioned by the present 
state of our best instruments, remains for the determination of 
those who are possessed of them, and are accustomed to use 
them.” 
LIV. Conjectures concerning the Cause, and Observations upon 
the Phenomena, of Earthquakes; particularly of that great 
Earthquake of the first of November 1755, which proved so 
fatal to the City of Lisbon, and whose Effects were felt as 
far as Africa, and more or less throughout almost all Europe ; 
by the Rev. Joun Micuu.r, M.A. Fellow of Queen’s College, 
Cambridge. 
[Concluded from p. 270.] 
Part III.—Sxrcrion I. a 
65. Iw the former part of this tract, I supposed a part of the 
roof.over some subterraneous fire to fall in: this is an event that 
cannot happen merely accidentally; for so long as the roof rests 
on the matter on fire, no part of it can fall in, unless the mat- 
ter below could rise and take its place: now, it is very diffi- 
cult to conceive how this should happen, unless it was to rise by 
, some 
