attraction of Schehallien. The conclusion thence
derived, that our globe weighs 4-1/2 times as much as an equal bulk of
water,[904] was not very exact. It was considerably improved upon by
Cavendish, who, in 1798, brought into use the "torsion-balance"
constructed for the same purpose by John Michell. The resulting estimate
of 5.48 was raised to 5.66 by Francis Baily's elaborate repetition of
the process in 1838-42. From experiments on the subject made in 1872-73
by Cornu and Baille the slightly inferior value of 5.56 was derived; and
it was further shown that the data collected by Baily, when corrected
for a systematic error, gave practically the same result (5.55).[905] M.
Wilsing's of 5.58, obtained at Potsdam in 1889,[906] nearly agreed with
it; while Professor Poynting, by means of a common balance, arrived at a
terrestrial mean density of 5.49.[907] Professor Boys next entered the
field with an exquisite apparatus, in which a quartz fibre performed the
functions of a torsion-rod; and the figure 5.53 determined by him, and
exactly confirmed by Dr. Braun's research at Mariaschein, Bohemia, in
1896,[908] may be called the standard value of the required datum.
Newton's guess at the average weight of the earth as five or six times
that of water has thus been curiously verified.

Operations for determining the figure of the earth were carried out during
the last century on an unprecedented scale. The Russo-Scandinavian arc,
of which the measurement was completed under the direction of the elder
Struve in 1855, reached from Hammerfest to Ismailia on the Danube,
a length of 25 deg. 20'. But little inferior to it was the Indian arc,
begun by Lambton in the first years of the century, continued by Everest,
revised and extended by Walker. Both were surpassed in compass by the
Anglo-French arc, which embraced 28 deg.; and considerable segments of
meridians near the Atlantic and Pacific shores of North America were
measured under the auspices of the United States Coast Survey. But these
operations shrink into insignificance by comparison with Sir David
Gill's grandiose scheme for uniting two hemispheres by a continuous
network of triangulation. The history of geodesy in South Africa began
with Lacaille's measurements in 1752. They were repeated and enlarged in
scope by Sir Thomas Maclear in 1841-48; and his determinations prepared
the way for a complete survey of Cape Colony and Natal, executed during
the ten years 1883-92 by C