168 BRUSH— A KINETIC THEORY OF GRAVITATION. 



Review and Extension of Gravity Pendulum Experiments 



Described in First Paper. 



Most of this work has been purposely delayed until after publica- 

 tion of the first paper,^ in the hope that if any source of large error 

 had been overlooked at the time of writing that paper, it would be 

 pointed out by some one. Nothing of this sort has been brought to 

 my attention. 



The next four or five pages, as far as Table I., are in substance 

 quoted from the first paper. 



Plate I. shows the pendulum apparatus as originally installed, to- 

 gether with driving clocks at the top, added later for long-continued 

 observations. 



A starting cradle, moving in guides on the low table just below 

 the cylindrical zinc and bismuth bobs, serves to start the pendulums 

 swinging exactly together in any desired amplitude. After pushing 

 the bobs sufficiently to the left, the cradle is suddenly withdrawn to 

 the right, leaving the bobs free. This device is entirely satisfactory 

 in performance. 



A horizontal thick plate of hardened steel is very firmly bolted to 

 the lower flange of a heavy iron I beam imbedded in the masonry of 

 the ceiling and walls of the room. The plate is dropped 6.5 cm. 

 below the beam by cylindrical iron spacers through which the bolts 

 pass, and is carefully leveled. Near one edge of the upper face of 

 the plate is a long shallow \/^ groove of 90° angle, with a slightly 

 rounded bottom carefully ground straight and polished after the plate 

 was hardened. 



From this plate hang two exactly similar pendulums of about 

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

 except for a few centimeters 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 hori- 

 zontal face of a 60° triangular "knife-edge" of hardened steel 

 1 Prog. Am. Phil. Soc, Vol. LX., No. 2, 1921. 



