BE USH— KINETIC THEORY OF GRAVITATION. 59 



the same always when the zincs and bismuths are exchanged. This 

 adjustment is made by gently tapping with a very small hammer one 

 or both projecting ends of the rod c, or driving either cylinder, as 

 indicated, further onto its rod with a block of soft wood until the 

 desired over-all dimension is obtained, indicating a standard (though 

 much smaller) double radius. In making this adjustment for the 

 following experiments the most patient care was exercised, and a 

 micrometer caliper of high precision was used. 



The driving clock had been running about two months since 

 winding when the experiments hereafter detailed were made, yet the 

 amplitude of the new pendulums was much greater than necessary 

 to unlock the escapement. One tooth of the " scape-wheel " was 

 marked, and this came round to a certain definite position every five 

 minutes by the face hands of the clock, indicating 40 gyrations of 

 the pendulum. Thus the clock was used as an accurate and de- 

 pendable counter of pendulum gyrations. A good watch was kept 

 always in the same position beside the clock to measure elapsed time, 

 and always wound at the beginning of an experiment. Room tem- 

 perature remained nearly constant throughout the experiments. A 

 glass hood always covered the clock to keep out air drafts. Runs 

 of 22 to 24 hours were usually made, to average up irregularities in 

 watch and clock rates. Total elapsed time in seconds was divided 

 by the counted number of gyrations to get the periods in seconds of 

 a single gyration. 



Many preliminary experiments were made to detect lack of 

 homogeneity in any cylinder, if such existed. To this end each 

 cylinder was separately turned about — its inner element made the 

 outer — or its upper end the lower. In no case was there any ob- 

 servable change of period. The matter of unequal air resistance of 

 the two pairs of cylinders was gone into quite extensively. Of 

 several attempts to equalize this, the following was found the most 

 trustworthy. A pair of short hollow cylinders of thin firm paper, 

 closed at one end, and of such diameter as to slip snugly on the 

 upper ends of the bismuth cylinders were prepared. These cylinders 

 or caps were pushed just far enough down on the bismuth cylinders 

 to make them equal the zinc cylinders in height, plus thickness of 



PROC. AMER. PHIL. SOC. VOL. LX, E, DEC. 20, I921. 



