176 BRUSH— A KINETIC THEORY OF GRAVITATION. 



true explanation, it is exceedingly interesting in showing that physical 

 condition affects the relation of weight and mass in bismuth, and 

 presumably in other substances. Certain other experiments on very 

 different lines, which will be detailed in a future paper, appear to 

 support this view. 



But the old and new sine cylinders, when compared with similar 

 care, also showed a similar, though smaller, difference in behavior, 

 the new zinc being faster than the old. This may be due to the fact 

 that the new bob was made of electrolytic zinc, presumably almost 

 chemically pure, while the old one was made of ordinary commercial 

 zinc, presumably not nearly so pure. 



The latter and subsequent experiments were greatly facilitated by 

 using a reading telescope located in the plane of the pendulum wires 

 when at rest, and focussed sharply on the nearer wire just above the 

 brass rod which carries the bob. This leaves the farther wire hazy 

 in outline, so that the two wires are easily distinguished as they cross 

 the field of view. When the pendulums are in synchronism, the wires 

 are superposed as they cross the center of the field. In this way 

 very small departures from synchronism can be detected in a few 

 minutes, which would require many times longer to grow sufficiently 

 to be seen with the unaided eye. 



Next, both zinc cylinders were mounted on one pendulum rod, the 

 new one above ; and both bismuth cylinders on the other rod, the new 

 one above, all as shown in Plate VIII. (the plate, however, showing 

 much shorter pendulum rods). The old bismuth cylinder had its 

 small paper vanes turned normal to line of swing, while the new 

 bismuth cylinder carried a very thin aluminum vane normal to swing, 

 of computed area sufficient to make its air resistance equal that of 

 the new zinc cylinder. 



Here we have bobs of double the former weight, and very different 

 in shape. Of course, the periods are slightly shortened. 



Then these double bobs were fully compared in the same manner 

 as were the former single bobs. The closely same results are shown 

 in Table II. 



Both pendulum rods were next shortened nearly a meter, and a 

 suitably higher table was placed below them, as shown in Plate VIII. 



