408 
SECOND REPORT— 1832 . 
entered into by Sir J. Herschel, how far the secular diminu¬ 
tion of the eccentricity of its orbit may have tended to the de¬ 
crease of its temperature; and the like. Now one of these 
points, and that very important to geological theory, appears 
to me to require further investigation ; I mean the conclusions 
fairly deducihle from the known density of the earth, as to the 
solid structure and composition of its interior. As its density 
is known to be considerably greater than that of a solid sphere 
composed of any such rocks as we are acquainted with (granite, 
for instance,) our primd facie inference would naturally be, 
that the interior is solid ; and that heavier materials than our 
ordinary rocks (such as metalliferous masses,) enter into its con¬ 
stitution. But to this it may be objected that the rocks alluded 
to have in themselves a principle of elasticity and compressi¬ 
bility, and therefore may, under the vast pressure existing in 
the interior of the globe, be condensed to such a degree as is 
far more than sufficient to account for the excess of the earth’s 
density, as compared with their specific gravity, and thus still 
to allow for considerable vacuities. To this, however, a coun¬ 
ter argument may be fairly adduced ; that, as the resistance to 
further compression increases with every additional pressure, 
that resistance may soon, in the case of these rocks, become 
practically infinite. A more accurate examination of the whole 
circumstances of this problem appears highly desirable *. 
The next branch to which I would call your attention may, 
the enormous abrasion of the solid matter of such immensely protuberant con¬ 
tinents as would, on that supposition, be left, by the violent and constant fluc¬ 
tuation of an unequilibriated ocean, would (according to an ingenious remark 
of Professor Playfair,) no doubt, in lapse of some ages, remodel the surface to 
the spheroidal form ; but the lunar theory teaches us that the internal strata, 
as well as the external outline of our globe, are elliptical, their centres being 
coincident, and their axes identical with that of the surface,—a state of things 
incompatible with a .subsequent accommodation of the surface to a new and dif¬ 
ferent state of rotation from that which determined the original distribution of 
the component matter. 
* I am well aware of Prof. Leslie’s ingenious Essay on this subject, in which 
he concludes that compressed light occupies the hollow centre of our globe, being 
the only substance which possesses a sufficient elastic force to resist and balance 
the enormous pressure of a vertical column of 3,500 miles; so that the interior 
of our planet is not, as falsely supposed, obscure, but filled with luminous aether, 
the most pure, concentrated, and resplendent. I will only observe, with re¬ 
ference to the calculations which are supposed to lead to this truly brilliant 
conclusion, that it is assumed that the modulus of compressibility of air, water, 
Src., is invariable, however greatly the pressure may be increased, so thatwe may 
have air condensed first into the density of water, and then, still following its 
former law and not that of the fluid whose place it has taken, to marble, and so 
on ad infinitum. In answer to this I shall only inquire whether it is actually 
found that the condensed gases, the subject of Faraday’s beautiful experiment, 
after they are metamorphosed into fluids still retain the same modulus ol com¬ 
pressibility which originally belonged to them in their gaseous state ? 
