Interferometer to the Horizontal Pendulum. 513 



I may add in conclusion that the experiments referred to 

 above, for measuring the gravitational attraction of two identi- 

 cal brass discs, led to curious results. It is easily seen that for 

 constant mass, the attraction of nearly contiguous discs should 

 increase roughly as the fourth power of their radius. For 

 discs 20 cm in diameter, however, the result is an invariable 

 repulsion, several times as large as the estimated gravitational 

 attraction, the position of equilibrium being reached gradually 

 in the lapse of several minutes. Since the discs are identical 

 and in metallic connection, it would seem as if the absolute 

 electric voltaic contact potential of the metals is being measured. 

 But this effect would be at least 100 times smaller than 

 the observed value. Similarly, the gravitational pressure 

 increment of the film of air between the discs is too small, 

 so that the cause of systematic repulsion in the hundreds of 

 experiments made is yet to be learned. 



Brown University, Providence, E. I. 



