With regard to the determination of the Mass and Density of the Earth
by direct experiment, we have, in addition to the deviations of the
pendulum produced by mountain masses, the variation of the same
instruments when placed in a mine 1200 feet in depth. The most recent
experiments were conducted by Professor Airy, in the Harton coal-pit,
near South Shields:[10] the oscillations of the pendulum at the bottom
of the pit were compared with those of a clock above; the beats of the
clock were transferred below for comparison by an electrio wire; and it
was thus determined that a pendulum vibrating seconds at the mouth of
the pit would gain 2¼ seconds per day at its bottom. The final result
of the calculations depending on this experiment, which were published
in the _Philosophical Transactions_ of 1856, gives 6·565 for the mean
density of the earth. The celebrated Cavendish experiment, by means
of which the density of the earth was determined by observing the
attraction of leaden balls on each other, has been repeated in a manner
exhibiting an astonishing amount of skill and patience by the late Mr.
F. Baily.[11] The result of these experiments, combined with those
previously made, gives as a mean result 5·441 as the earth’s density,
when compared with water; thus confirming one of Newton’s astonishing
divinations, that the mean density of the earth would be found to be
between five and six times that of water.
Humboldt is, however, of opinion that “we know only the mass of
the whole earth and its mean density by comparing it with the
open strata, which alone are accessible to us. In the interior of
the earth, where all knowledge of its chemical and mineralogical
character fails, we are limited to as pure conjecture as in the
remotest bodies that revolve round the sun. We can determine
nothing with certainty regarding the depth at which the geological
strata must be supposed to be in a state of softening or of liquid
fusion, of the condition of fluids when heated under an enormous
pressure, or of the law of the increase of density from the upper
surface to the centre of the earth.”--_Cosmos_, vol. i.
In M. Foucault’s beautiful experiment, by means of the vibration of
a long pendulum, consisting of a heavy mass of metal suspended by a
long wire from a strong fixed support, is demonstrated to the eye
the rotation of the earth. The Gyroscope of the same philosopher
is regarded not as a mere philosophical toy; but the principles of
dynamics, by means of which it is made to demonstrate the earth’s
rotation on its own axis, are explained with the greatest clearness.
Thus the ingenuity of M. Foucault, combined with a profound knowledge
of mechanics, has obtained proofs of one of the most interesting
problems of astronomy from an unsuspected source.