43 (with a probable error of only 0.0233), a result which, being increased
by the quantity by which the Earth's centrifugal force diminishes the force
of gravity for the latitude of Freiberg (50 degrees 55'), becomes changed to
5.44. The employment of cast iron instead of lead has not presented any
sensible difference, or none exceeding the limits of errors of observation,
hence disclosing no traces of magnetic influences. (Reich, 'Vrsuche uber
die mittlere Dichtigheit der Erde', 1838, s. 60, 62, and 66.) By the
assumption of too slight a degree of ellipticity of the Earth, and by the
uncertainty of the estimations regarding the density of rocks on its
surface, the mean density of the Earth, as deduced from experiments on and
near mountains, was found about one sixth smaller than it really is, namely,
4.761 (Laplace, 'Mecan. Celeste', t. v., p. 46), or 4.785. (Eduard Schmidt,
'Lehrb. der Math. Geogr.', bd. i., 387 und 418.) On Halley's hypothesis of
the Earth being a hollow sphere (noticed in page 171), which was the germ of
Franklin's ideas concerning earthquakes, see 'Philos. Trans.' for the year
1693, vol. xvii., p. 563 ('On the Structure of the Internal Parts of the
Earth, and the concave habited 'Arch of the Shell'). Halley regarded it as
more worthy of the Creator "that the Earth, like a house of several stories,
should be inhabited both without and within.
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