Physical geography. By Mary Somerville ...

CHAP. I. LO AL DI S T U R BAN CE S. 19
The discrepancies in the results, from the comparison of the different sets of pendulum experiments, and also of degrees of the meridian, arise from local attraction, as well as from irregularities in the
form of the earth. These attractions, arising from dense masses of
rock in mountains, cause the plumb-line to deviate from the vertical,
and when under ground they alter the oscillations of the pendulum.
Colonel Sabine, who made experiments with the pendulum from the
equator to within ten degrees of the north pole, discovered that the
intensity is greatly augmented by volcanic islands. A variation to
the amount of a tenth of a second in twenty-four hours can be perfectly ascertained in the rate of the pendulum; but from some of
these local attractions a variation of nearly ten seconds has occurred
during the same period. The islands of St. Helena, Ascension, St.
Thomas, the Isle of France, are some of those noted by Colonel Sabine.
There are other remarkable instances of local disturbance, arising
from the geological nature of the soil; for example, the intensity of
gravitation is very small at Bordeaux, from whence it increases rapidly to Clermont-Ferrand, Milan, and Padua, where it attains a
maximum (owing, probably, to dense masses of rock under ground),
and from thence it extends to Parma. In consequence of this local
attraction, the degrees of the meridian in that part of Italy seem to
increase towards the equator through a small space, instead of decreasing, as if the earth were drawn out instead of flattened at the
poles.
It appears from this, that the effect of the whole earth on a pendulum or torsion balance may be compared with the effect of a small
part of it, and thus a comparison may be instituted between the mass
of the earth and the mass of that part of it. Now a leaden ball was
weighed against the earth by comparing the effects of each upon a
balance of torsion; the nearness of the smaller mass making it produce a sensible effect as compared with that of the larger, for by the
laws of attraction the whole earth must be considered as collected in
its centre; in this manner a value of the mass of the earth was obtained, and, as its volume was known, its mean density was found to
be 5-675 times greater than that of water at the temperature of 62~
of Farenheit's thermometer. Now, as that mean density is double
that of basalt, and more than double that of granite, rocks which
undoubtedly emanate from very great depths beneath the surface of
moon on the protuberant matter at the earth's equator does not actually
1 1
give the compression, it proves that it must be between -- and -. Coin279 -573
cidences so near and so remarkable, arising from such different methods,
show how nearly the irregular figure of the earth has been determined.
The inequalities in the motions of the moon and earth alluded to are explained in Sections 5 and 11,' Connexion of Physical Sciences.'