Nipher — Gravitation and Electrical Action. 169 



a modified form of that used by Cavendish. The sus- 

 pended masses consisted of two lead tubes, each being 

 about 15 cm. in length, each having a mass of 50 grams. 

 They were mounted around the ends of a brass tube 91 

 cm. in length, and having a mass of 30 grams. This tube 

 was suspended upon two loosely twisted threads of silk 

 fibers, 180 cm. in length, whose distance apart was ap- 

 proximately 0.4 cm. The twist was removed from these 

 threads by hanging upon each a mass of 65 grams. This 

 formed a very sensitive bifilar suspension. The sus- 

 pended masses were wholly surrounded by a metal shield 

 of rectangular form 10x12.5 cm. in cross section. The 

 suspension fibers were enclosed in a metal tube, having 

 a torsion head at the top, thus providing means for prop- 

 erly adjusting the position of the brass tube. The ends 

 of the rectangular shield were provided with metal caps, 

 fitting closely into its ends. They entered the shield a 

 distance of 2.5 cm. An opening at the middle of the 

 shield, in front of a mirror mounted upon the suspen- 

 sion wire, served for observation of position by means 

 of a telescope and scale. This opening was covered by 

 a sheet of glass which was sealed to the shield by means 

 of sealing-wax. The window was covered with a metal 

 wire screen having about 5 wires per running cm. The 

 mirror was observed through this wire screen, the tele- 

 scope being focused upon the scale at the telescope. A 

 change of one scale division represented a change of 2.36 

 minutes of arc in the position of the suspended tube. The 

 suspended masses and the brass tube on which they were 

 mounted were surrounded by sheets of asbestos paper, 

 which fitted into the caps at the ends of the shield, and 

 fitted loosely the interior of the shield. These two lin- 

 ings formed extensions of the end caps and were in- 

 tended to prevent a convection of the air within the shield. 

 The large masses each consisted of two fifty-pound 

 weights placed one upon the other and mounted upon 

 heavy columns of rubber. They were separated from the 

 metal shield by a space of about 2 cm. In this space was 



