BRUSH— A KINETIC THEORY OF GRAVITATION. 171 



materially longer than the zinc one. It was the whole aim of the 

 pendulum experiments to detect and measure this difference if it 

 existed. 



Next, the weights were exchanged, so that in effect the bismuth 

 pendulum was now the shorter one by double the former difference. 

 On again starting the pendulums, at the former amplitude, loss of 

 synchronism was easily observable in 2 minutes — the bismuth gaining. 

 In half an hour the bismuth gain was very large. In the same and 

 other forms this experiment was repeated many times, and always 

 with the same unequivocal result. 



Equality of air resistance was effected by attaching small paper 

 vanes to opposite sides of the bismuth normal to the line of swing, 

 of such size as to produce air damping equal to that of the zinc as 

 shown by equal time loss of amplitude. The necessary vanes are very 

 small. 



It appears from this experiment that the earth's gravitation field, 

 which is here the accelerating force, grips the bismuth more strongly 

 per unit of mass than it grips the zinc per unit of mass ; in other 

 words, a given mass of bismuth appears to zveigh more than the same 

 mass of zinc. Apparently the length of a standard seconds pendulum 

 may depend on the material of which it is made. 



The greater diameter of the zinc cylinder slightly lowers its center 

 of oscillation; and this accounts for about 10 per cent, of the effect 

 above described, as determined by elaborate experimentation which 

 need not be detailed here, and which was approximately verified by 

 computation. 



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

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

 adapted to drive the pendulums continuously at an amplitude of 

 13 cm. 



After synchronizing the zinc and bismuth pendulums at this ampli- 

 tude, the zinc and bismuth weights were exchanged as heretofore 

 described. Then they were started exactly together and allowed 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 



