BRUSH— KINETIC THEORY OF GRAVITATION. 53 



2.284 m. effective length and 15.2 cm. apart. Each pendulum rod, 

 except for a few cm. at each end, is of mild steel, perfectly straight 

 and 1.6 mm. diam. Both rods were cut from the same specimen, 

 so as to have the same temperature coefficient. The upper 20 cm. 

 of each rod is 0.4 cm. diam. round steel with fine screw thread and 

 thumb nut on its upper part. The thumb nut has eight radial holes 

 for a long brass pin, the whole adapted to effect very fine adjustment 

 of pendulum length. The thumb-nut rests on the horizontal face of 

 a 60° triangular " knife-edge " of hardened steel through which the 

 rod passes. The upper part of the rod is slightly flattened on one 

 side by grinding, and a thumb-screw in one end of the knife-edge 

 block bears against the flattened side of the rod and clamps it firmly 

 in the block after each length adjustment is made. The knife-edge, 

 ground true and sharp, rests in the plate groove above described, 

 while the rod passes downward through an opening in the side of 

 the plate. 



Each pendulum rod terminates at its lower end in a straight 

 brass rod 13 cm. long and 0.4 cm. diam. A perfectly straight hori- 

 zontal steel pin passes loosely through the brass rod near its lower 

 end, and on this pin the cylindrical bob, or weight as I shall here- 

 after call it, rests. 



'Fig. 2 shows the upper and lower parts of one pendulum in 

 detail, with the bismuth weight in place. 



The brass rod at the lower end passes just freely through the 

 weight, and accurately in its axis. A weight is easily removed from 

 either pendulum by lowering it after the pin is withdrawn, and an- 

 other weight may be substituted by reversing the procedure. While 

 this is being done the pendulum rod is kept taut by another tem- 

 porary, radially slotted, lead weight applied just above, and resting 

 on the upper end of the brass rod. Thus the weights forming the 

 bobs of the two pendulums may readily be exchanged without dis- 

 turbing anything else. 



The weights to be compared, bismuth and zinc in the first in- 

 stance, were made very accurately the same in height, and with upper 

 and lower ends as nearly plane and parallel as possible, by careful 

 grinding on a perfectly flat surface. 



