56 BRUSH— KINETIC THEORY OF GRAVITATION. 



A pair of high-grade, weight-driven clock movements were next 

 added to the apparatus, as shown in the upper part of Plate VI., and 

 adapted to drive the pendulums continuously at an amplitude of 

 13 cm. 



After synchronizing the zinc and bismuth pendulums at this 

 amplitude, the zinc and bismuth weights were exchanged as hereto- 

 fore described. Then they were started exactly together and al- 

 lowed to run until they were again exactly together, the bismuth 

 having thus gained two full beats. Half the elapsed time was taken 

 as the value of one beat gain. 



Again the pendulums were synchronized, the zinc weight now 

 being on the pendulum formerly occupied by the bismuth weight ; 

 then the weights were exchanged as before, the pendulums started 

 together, and allowed to run until the bismuth had gained two beats 

 as formerly. This procedure was for the purpose of verifying the 

 first finding and to expose any considerable difiference there might 

 be in the performance of the driving clocks. No such difference 

 was found ; yet for verification the same procedure was followed in 

 the next experiments. 



A cylinder of very pure iron was next prepared, of exactly the 

 same height, and approximately the same weight as the zinc and 

 bismuth cylinders, and adjusted for center of gravity with the same 

 care. 



The iron weight or cylinder was then compared with the zinc 

 weight and with the bismuth weight, with the same care used in 

 comparing the zinc and bismuth as above described. The iron gave 

 results intermediate between those of zinc and bismuth, rather nearer 

 the zinc. 



Table II. shows the performance of the zinc-iron, the iron-bis- 

 muth and the zinc-bismuth combinations. The measurements of 

 time required to gain one beat check and confirm each other re- 

 markably well. 



As the pendulums make about 2,388 oscillations per hour, the 

 bismuth gains one beat, or oscillation, in about 17.432; but as before 

 pointed out, the real zinc-bismuth effect is only half of this, say one 

 part in 35.000. This weight-mass difference effect, though not large, 

 appears fairly well established and is impressive. 



