304 



REPORT — 1893. 



The support of the pendulum (fig. 7) is arranged symmetrically with 

 respect to the diameter of the tripod-stand, which passes throuo-h the 

 third foot-screw. It consists of a strong rectangular frame, A, the plane 

 of which is parallel to the lens above mentioned (whose position is 

 shown by the circle in the figure), and is screwed to the base of the 

 tripod-stand behind the lens. It carries two horizontal cylinders, B, h, 

 one lying vertically over the othei', which can be turned round their 

 axes with considerable friction. In the middle of the cylinders are very 

 fine steel points, c,c (shown projecting upwards in the figure). These 

 are screwed to the cylinders, so that they are perpendicular to their 

 axes, and project only a little way from their surfaces. The vertical 

 distance between the axes of the cylinders is 68 mm. 



Ftg. 7. 



The pendulum is made completely of brass in the form of an isosceles 

 triangle (fig. 8). D, E, F are thin tubes ; H, a small weight attached at 

 the vertex of the triangle. At p and Q are two small spherical agate 

 cups, 2-5 mm. in radius, their centres being 68 mm. apart. The steel 

 points against which the cups rest also project from the axes of the 

 cylinders by 2-5 mm., so that in every position the points are perpendi- 

 cular to the tangent-planes to the cups. The rod, K, which projects per- 

 pendicularly from the tube, f, has a perforation, l, in the direction p q 

 for the reception of a knife-edge, on which the pendulum can be sus- 

 pended in a vertical position. Ji is a mirror which projects through the 



