170 BRUSH— A KINETIC THEORY OF GRAVITATION. 



either pendulum by lowering it after thv, pin is withdrawn, and an- 

 other weight may be substituted by reversing the procedure. While 

 this is being done the pendulum rod is kept taut by another tempo- 

 rary, radially slotted, lead weight applied just above, and resting on 

 the upper end of the brass rod. Thus the weights forming the bobs 

 of the two pendulums may readily be exchanged without disturbing 

 anything else. 



The weights to be compared, bismuth and zinc in the first instance, 

 were made very accurately the same in height, and with upper and 

 lower ends as nearly plane and parallel as possible, by careful grinding 

 on a perfectly flat surface. 



It is essential that the centers of gravity of the weights be exactly 

 the same distance above their supporting pins. To assure this, each 

 weight was adjusted to have its center of gravity exactly midway 

 between its upper and lower ends by the following procedure : The 

 pendulums having been started swinging with a definite amplitude 

 and brought to synchronism by length adjustment, one of the weights 

 was turned over ; this at first resulted in loss of synchronism at the 

 same amplitude. Then, as indicated, the upper or lower cylindrical 

 portion was slightly reduced in diameter by turning oft' or sand- 

 papering in the lathe. Again the pendulums were synchronized, and 

 again the same weight was turned over and synchronism tested. 

 This process was repeated again and again with each weight until 

 either could be turned over without affecting synchronism in the 

 slightest observable degree. In making these adjustments very 

 minute departure from synchronism could be detected in half an hour 

 at the turning points of the swing. 



Instead of making the cylinders the same in diameter, they were 

 made approximately the same in zt'cigJif, about 1.377 ^S-' so that 

 when they were exchanged the length of the pendulum rods would 

 not be affected. Otherwise it would have been necessary to apply 

 corrections for the elastic modulus of the rods and for their effect on 

 the center of oscillation of the zi'ciglits with every exchange. The 

 latter correction would have been very large, and liable to error. 



Finally, the zinc and bismuth pendulums were adjusted to syn- 

 chronism as perfectly as possible in 40-minute runs with initial ampli- 

 tude of 35 cm. As it turned out, the bismuth pendulum was then 



