bo 
54 The Rev. John Michell’s Opinions 
influence on the mean temperature of the mixture; therefore, 
unless the specific gas be known, the specific heat cannot be de- 
termined from the mean temperature. 
XLI. 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 ; 
ly the Rev.Joun Micuext, M.A. Fellow of Queen’s College, 
Cambridge. 
[Continued from p. 195.] 
37. Luss frequency of subterraneous fires, in the neighbour- 
hood of volcanos, will appear still more probable, if we consider 
the internal structure of the Earth; and, as it will be necessary 
also, in order to understand what follows, to know a little more 
of this matter, than what falls under common observation, I shall 
endeavour to give the reader some account of it. 
38. The Earth then (as far as one can judge from the ap- 
pearances) is not composed of ‘heaps of matter casually thrown 
together, but of regular and uniform strata. These strata, 
though they frequently do not exceed a few feet, or perhaps a 
few inches, in thickness, yet often extend in length and breadth 
for many miles, and this without varying their thickness consi- 
derably. The same Stratum also preserves a uniferm character 
throughout (a), though the strata immediately next to each other 
are very often totally different. Thus, for instance, we shall have, 
perhaps, a stratum of potters clay; above that, a stratum of coal ; 
then another stratum of some other kind of clay; next, a sharp 
grit sand-stone; then clay again; next, perhaps, sand-stone 
again; and coal again above that: and it frequently happens, 
that none of these exceed a few yards in thickness. There are, 
however, many instances, in which the same kind of matter is 
extended to the depth of some hundreds of yards; but in all 
these, a very few only excepted, the whole of each is not one 
continued mass, but is again subdivided into a great number of 
thin lamine(l), that seldom are more than one, two, or three feet 
thick, and frequently not so much. ! 
39. Be- 
(a) Itis not clear, that Mr. Michell in this passage, meant to speak strictly, 
or of mineralogical characters only ; see my note on § 47, and the 9th Item 
of Mr. Smith’s Claims, Phil. Mag. vol. li. p. 177.—J. FAREY. 
(b) It does not clearly appear, whether Mr, M. here speaks exclusively of 
thin Strata, or beds parallel or nearly so, to the general stratification; or 
whether 
