308 Royal Astronomical Society. 
ducing the effect of attraction, and transmitting its influence through 
all the screens which were employed to exclude disturbing causes, 
with as much ease as through space occupied only by air. 
It had been previously shown by Laplace, that the mean 
density of the earth must exceed that of the sea, in order to pre- 
serve the stability of the oscillations of the latter: and subse- 
quently Poisson, in a paper contained in the seventh volume of the 
Memoirs of the Institute, concludes (taking Cavendish’s value of 
the mean density as one of the data, and applying the forces with 
which the sun and moon act in causing precession, in a manner 
which is perfectly just, though open to a small amount of objection 
against the numerical accuracy of the remaining data) not only 
that there must be an increase of density from the circumference 
towards the centre, but that, even on the supposition of an homo- 
geneous nucleus, the variable strata which enwrap this nucleus 
must extend to a depth of at least one-fourth of the radius. The 
physical astronomer, using that power of judging which the study 
of past discovery has made habitual to him, has long considered 
it as all but proved that our terrestrial globe is not only solid to 
the centre, or at least having its interior “strata composed of fluid 
with the density of a solid, but also that the density of successive 
strata is gradually increasing from the surface to the centre itself. 
But those who have not studied physics, and even experimental 
philosophers of note who have not paid particular attention to the 
theory of gravitation, have frequently taken the hypothesis of a 
hollow globe as_ being that which has the highest probability in its 
favour. The late Professor Leslie, for example, takes the hollow- 
ness of our earth for granted, and proceeds to reasun~ upon the 
manner in which the part of the interior which is destitute of solid 
matter is filled. It is his opinion that it is a sure result of induc- 
tion*, that ‘‘ the great central concavity is not that dark and dreary 
abyss which the fancy of poets had pictured. On the contrary, this 
spacious internal vault must contain the purest ethereal essence, 
Light in its most concentrated state, shining with intense refulgence 
and overpowering splendour.” It is not my intention to discuss the 
grounds upon which, taking comparative hollowness for granted, the 
void is to be supposed filled with light or any other zether, but only 
to observe that the experiment before us strikes most completely at 
the preliminary assumption. Maskelyne made the earth to be a 
ball five times as dense as the same bulk of water; Cavendish, five 
times and a half; but Mr. Baily, whose experiments are far more 
numerous and well supported, five times and two-thirds. The solid 
rocks with which the geologist professes that his knowledge of the 
interior strata of our earth terminates are only between three and 
four times as heavy as water: barytes itself, which takes itsname from 
the great density of its compounds, has one of the heaviest of those 
compounds, the sulphate, under four times and a half its weight of 
water, If we were to ask what substance must be chosen, so that 
* Elements of Natural Philosophy, vol. i. p. 453. 
