118 Proceedings of the Royal Physical Society. 
and by Mr Fox in the Cornish mines. These observations 
were collected and arranged by M. Cordier in an admirable 
essay published in the " Mem. du Mus. d'Hist. Naturelle" 
for 1827, in addition to several experiments of his own ; 
and after rejecting two-thirds of the observations as of 
doubtful value, it was ascertained that below^ the " stratum 
of invariable temperature" there was a constant rate of in- 
crease from above downwards of 1° Fahr. for about every 
45 feet of descent. Subsequent researches have shown that 
the rate of increase is 1° Fahr. for every 60 feet of descent. 
All the observations, whether on the air, the water, or the 
rock of subterranean cavities, agree in this general law\ 
These experiments are therefore justly regarded as afford- 
ing positive proofs of an internal heat. 
VL The Configuration of the Earth. — Mathematicians 
assert that the form of the earth is just what it should be, 
provided it had been originally fluid ; its equatorial diameter 
exceeding its polar diameter by about 27 miles. In con- 
nection with this argument in support of the theory of a 
central heat, it will be necessary to allude to the bold specu- 
lation of Sir W. Herschel, La Place, and other modern 
philosophers, in regard to the formation of planetary bodies. 
This speculation depends upon certain sublime and as yet 
unexplained discoveries in astronomy. It was observed 
that many of the fixed stars, when examined by the tele- 
scope, appeared like brilliant points surrounded by an ex- 
tensive nebulous atmosphere. Other self-luminous spots or 
patches of vapour have been seen varying in brightness in 
different parts, and having various forms of outline, and to 
these luminous appearances the term of nebulae has been 
applied. Herschel at first conjectured that the nebulas 
were composed of stars at incalculable distances, and that 
they might be resolved into individual stars by the aid of 
more powerful telescopes ; but he subsequently ascertained 
that the remarkable nebulae in the constellations of Orion 
and Andromeda, although brilliant enough to be seen by the 
unassisted eye, were utterly irresolvable even in his gigantic 
telescopes. From extended observations on the circular 
