52 BRUSH— KINETIC THEORY OF GRAVITATION. 



Undoubtedly the commonly accepted measure of mass, viz., its 

 resistance to a definite accelerating force, positive or negative, is 

 reliable and safe. 



If, then, we have two pendulums of exactly the same real length, 

 one with a zinc bob and the other with a bismuth bob, and find that 

 their periods are not the same, we may reasonably infer that the 

 accelerating force of gravity acts more strongly per unit of mass on 

 the one having the shorter period than on the other. This is the 

 principle, though not exactly the method, of the pendulum experi- 

 ments next described. 



It was realized from the start that the difiference, if any exists, 

 must be very small, and not easy to detect, because the earth's field 

 is so enormously preponderating that it does virtually all the attract- 

 ing; and the supposed differences in the extremely weak fields of the 

 zinc and bismuth may be so nearly lost in the immensity of the 

 earth's field as to be undetectable. Nevertheless, it was thought 

 worth while to try the experiment in view of the importance of the 

 subject. 



The photograph, Plate VI., shows the pendulum apparatus as 

 originally installed, together 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 the bobs sufficiently to the left, the cradle is suddenly withdrawn to 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 V grove 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 



