By a series of comparisons with Pendulums placed at the surface and
the interior of the Earth, the Astronomer-Royal has ascertained the
variation of gravity in descending to the bottom of a deep mine, as
the Harton coal-pit, near South Shields. By calculations from these
experiments, he has found the mean density of the earth to be 6·566,
the specific gravity of water being represented by unity. In other
words, it has been ascertained by these experiments that if the earth’s
mass possessed every where its average density, it would weigh, bulk
for bulk, 6·566 times as much as water. It is curious to note the
different values of the earth’s mean density which have been obtained
by different methods. The Schehallien experiment indicated a mean
density equal to about 4½; the Cavendish apparatus, repeated by Baily
and Reich, about 5½; and Professor Airy’s pendulum experiment furnishes
a value amounting to about 6½.
The immediate result of the computations of the Astronomer-Royal is:
supposing a clock adjusted to go true time at the top of the mine, it
would gain 2¼ seconds per day at the bottom. Or it may be stated thus:
that gravity is greater at the bottom of a mine than at the top by
1/19190th part.--_Letter to James Mather, Esq., South Shields._ See
also _Professor Airy’s Lecture_, 1854.