1865.] The Proposed Pendulum Operations for India. 267 
a kind of inverted stirrup ; in the part corresponding to the foot plate 
there was fixed a wheel with a sharp edge turning on its axis, This 
wheel was placed on a grooved plate and formed the knife edge for 
suspension ; the arrangements for observing were similar to Bessel’s. 
Corresponding observations, though not with the same apparatus, were 
taken by Biot and Mathien at Bordeaux. The result was a density 
of 4.95. 
One more attempt to determine the density of the earth, by means 
of the pendulum,.was made in 1854, by the Astronomer Royal, Professor 
Airy, at the Harton Colliery pit. Two invariable pendulums were set 
up in the same vertical line, one at the top, the other at the bottom of 
the pit ; and their coincidences with the pendulums of two clocks were 
simultaneously observed, the relative rates of the clocks being deter- 
mined by a galvanic apparatus. After each series of coincidences the 
pendulums were interchanged. The distance between the upper and 
lower pendulums was 1256 feet ; a careful description of the interven- 
ing strata was prepared and specimens submitted to Professor W. H. 
_ Miller who determined their specific gravities. The acceleration of 
the seconds’ pendulums below was 2.24 seconds per diem, and the 
resulting mean density of the earth was 6.565: 
The best value of the earth’s ellipticity as yet deduced from pendu- 
lum observations is undoubtedly that of Mr. Baily’s. He combined 
all the observations taken with invariable pendulums, and after apply- 
ing to them all corrections, obtained a mean ellipticity of st;.5. The 
latest value of the same, from geodetic observations, is Captain 
Clarke’s R. E. which includes the new Russian are and is 547.35. 
The ellipticity obtained from observations of precession and nutation is 
gigs (Aizy’s tracts). 
The apparatus for the Indian experiments consists of two invariable 
pendulums on Kater’s principle, a vacuum apparatus with air pump 
for exhausting, an astronomical clock by Shelton, a good battery of 
thermometers, and 4 transit instrument. Both pendulums have already 
done good service: one having been used by General Sabine in his 
extensive range of experiments, the other by Professor Airy in his 
Harton pit experiments ; they cannot be considered, however, to have 
retained their original length, as their knife edges have been reground. 
Each is composed of a bar of plate brass 1.6 inches wide and rather less 
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