58 



BRUSH— KINETIC THEORY OF GRAVITATION. 



and accurately the same in diameter — about 2.8 cm. Each cyhnder 

 has its ends ground as flat and parallel as possible and at right 

 angles with its axis. The members of each pair of cylinders have 

 very closely the same weight, and the combined weight of the bis- 

 muths differs from the combined weight of the zincs by a fraction 

 of a milligram only. The weight of each pair is 304.541 -f- grms. 

 Of course, the zinc cylinders are considerably higher than the bis- 

 muths on account of their lower specific gravity. 



Zn 



m 



K 





Fig. 3. 



It is essential that the axes of the cylinders be equi-distant from, 

 and parallel with, the axis of the member b. This adjustment is 

 first approximately made by means of a jig in which all the parts 

 are placed. At the left of the figure one of the cylinders is shown 

 in section, which also shows the rod c ground wedge-shaped for a 

 considerable part of its length within the cylinder. Into the wedge- 

 shaped spaces thus left long thin wedges of soft wood are driven, 

 the upper or lower one more than the other, until the axis of the 

 cylinder is parallel with the axis of b. The wedges also serve to 

 fasten the cylinder tightly onto the rod. After the final adjustments 

 the protruding parts of the wedges are broken off. The weight of 

 the wedges is small, and being virtually the same for each cylinder, 

 is negligible. 



The most important adjustment of all is the radii of gyration, 

 because an error here of only 0.03 mm. in both radii would double 

 or obliterate the whole difference found in the behavior of the zinc 

 and bismuth, though this difference, as will be seen, is very consider- 

 able. The aim is to make the double radius as accurately as possible 



