210 



RECENT STUDIES IN GRAVITATION. 



principle of forced oscillations by turning one sphere round and round 

 at a constant rate, so that the couple would act first in one direction 

 and then in the other, alternately, and so set the hanging sphere 

 vibrating to and fro. The nearer the complete time of vibration of 

 the applied couple to the natural time of vibration of the hanging 

 sphere, the greater would be the vibration set up. This is well illus- 

 trated b}^ moving the point of suspension of a pendulum to and fro in 

 gradually' decreasing periods, when the swing gets longer and longer, 

 till the period is that of the pendulum, and then decreases again; or, 



To /tccamt 



ReffuLcLtor 



FjG. 9. — Experiment on directive action of one quartz crystal on another. 



by the experiment of varying the length of a jar resounding to a given 

 fork, when the sound suddenly swells out as the length becomes that 

 which would naturall}^ give the same note as the fork. Now, in look- 

 ing for the couple between the crystals, there are two possible cases. 

 The most likely is that in which the couple acts in one way while the 

 turning sphere is moving from parallel to crossed, and in the opposite 

 way during the next quarter turn from crossed to parallel; that is, the 

 couple vanishes four times during the revolution, and this we may 

 term a quadrantal couple; but it is just possible that a quartz crystal 



