uence of the
hypothesis of primitive and simultaneous fluidity than proved by
experiment. The inequalities in the moon's motion, by which some have
endeavored to confirm them, are so extremely slight, that the opinion
can be regarded as little more than a probable conjecture.

The mean density of the earth has been computed by Laplace to be about
5-1/2, or more than five times that of water. Now the specific gravity
of many of our rocks is from 2-1/2 to 3, and the greater part of the
metals range between that density and 21. Hence some have imagined that
the terrestrial nucleus may be metallic--that it may correspond, for
example, with the specific gravity of iron, which is about 7. But here a
curious question arises in regard to the form which materials, whether
fluid or solid, might assume, if subjected to the enormous pressure
which must obtain at the earth's centre. Water, if it continued to
decrease in volume according to the rate of compressibility deduced from
experiment, would have its density doubled at the depth of ninety-three
miles, and be as heavy as mercury at the depth of 362 miles. Dr. Young
computed that, at the earth's centre, steel would be compressed into
one-fourth, and stone into one-eighth of its bulk.[745] It is more than
probable, however, that after a certain degree of condensation, the
compressibility of bodies may be governed by laws altogether different
from those which we can put to the test of experiment; but the limit is
still undetermined, and the subject is involved in such obscurity, that
we cannot wonder at the variety of notions which have been entertained
respecting the nature and conditions of the central nucleus. Some have
conceived it to be fluid, others solid; some have imagined it to have a
cavernous structure, and have even endeavored to confirm this opinion by
appealing to observed irregularities in the vibrations of the pendulum
in certain countries.

An attempt has recently been made by Mr. Hopkins to determine the least
thickness which can be assigned to the solid crust of the globe, if we
assume the whole to have been once perfectly fluid, and a certain
portion of the exterior to have acquired solidity by gradual
refrigeration. This result he has endeavored to obtain by a new solution
of the delicate problem of the processional motion of the pole of the
earth. It is well known that while the earth revolves round the sun the
direction of its axis remains very nearly the s