the true figure of the Earth is to a regular
figure as the uneven surfaces of water in motion are on the even surface of
water at rest.
When the Earth had been measured, it still had to be weighed. The
oscillations of the pendulum* and the plummet have here likewise served to
determine the mean density of the Earth, either in connection with
astronomical and geodetic operations, with the view of finding the
deflection of the plummet from a vertical line in the vicinity of a
mountain, or by a comparison of the length of the pendulum in a plain and on
the summit of an elevation, or, finally, by the employment of a torsion
balance, which may be considered as a horizontally vibrating pendulum for
the measurement of the relative density of neighbouring strata.
[footnote] *La Caille's pendulum measurements at the Cape of Good Hope,
which have been calculated with much care by Mathieu (Delambre, 'Hist. de
l'Astron. au 18me Siecle', p. 479), give a compression of 1/284.4th; but,
from several comparisons of observations made in equal latitudes in the two
hemispheres (New Holland and the Malouines (Falkland Islands), compared with
Barcelona, New York, and Dunkirk), there is as yet no reason for supposing
that the mean compression of the southern hemisphere is greater than that of
the northern.
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