BRUSH— KINETIC THEORY OF GRAVITATION. 55 



hour at the turning points of the swing. For certain reasons all 

 tests were made with the same initial amplitude. 



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

 made approximately the same in weight, about 1.377 kg., 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 weight 

 with every exchange. The latter correction would have been very 

 important, but liable to error. 



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

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

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

 then 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 gain- 

 ing. In 40 minutes 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 

 projections 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. 



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 weigh more than the 

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

 depends 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 verified by com- 

 putation. 



