288 



Professor John H. Poynting 



[Feb. 23, 



measured the attraction between the crystals and the lead when the 

 axes of the crystals were set in various positions. But the variation 

 in the attraction was merely of the order of error of experiment. In 

 another experiment the attracted masses were small calc spar crystal 

 cylinders weighing a little more than ^ gm. each. But again there 

 was no evidence of variation in the attraction with variation of axial 

 direction. 



f 



o" 



■:o/ 



Fig. 8. — Experiment on gravitative permeability 

 (Austin and Thvving). 



Practically the same problem was attacked in a different way by 

 Mr. Gray and myself.* We tried to find whether a quartz crystal 

 sphere had any directive action on another quartz crystal sphere close 

 to it, whether they tended to set with their axes parallel or crossed. 



It may easily be seen that this is the same problem by con- 

 sidering what must happen if there is any difference in the attraction 

 between two such spheres when their axes are parallel and when they 

 are crossed. Suppose, for example, that the attraction is always 

 greater when their axes are parallel, and this seems a reasonable 

 supposition, inasmuch as in straightforward crystallisation successive 

 parts of the crystal are added to the existing crystal, all with their 

 axes parallel. Begin, then, with two quartz crystal spheres near 

 each other with their axes in the same plane, but perpendicular to 



* Phil. Trans. 192, 1899, A, p. 245. 



