1865.] The Proposed Pendulum. Operations for India. 267 



formed of a heavy sphere Suspended by a wire, which was attached to 

 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 Boyal, 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' pendulum 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 T ~g-- -3. The 

 latest value of the same, from geodetic observations, is Captain 

 Clarke's, R. E. which includes the new Russian arc and is jgi.-j^. 

 The ellipticity obtained from observations of precession and nutation is 

 30V.3 (Airy'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 a 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. 

 35 



