BRUSH— A KINETIC THEORY OF GRAVITATION. 



179 



to line of swing, and as high and broad as a single cylinder, it would 

 double the air resistance of that pair. 



The diameters of the zinc and bismuth cylinders are, respectively, 

 72 and 61 mm., and their common height (single) 48 mm. Inci- 

 dentally it will be seen that a zinc cylinder has 18 per cent, greater 

 diameter than a bismuth cylinder, and presumably 18 per cent, more 

 air resistance. 



An air vane of thin cardboard 48 mm. high was placed on each 

 double pendulum bob as shown in Fig. 2; that on the zinc 50 mm. 

 wide, and on the bismuth 83 mm. wide, as shown by the dotted lines. 

 The vanes were adapted to be turned into the line of swing, which 



/-, n 



A-i n 



Fig. 2. 



was equivalent to removing them, or normal to the swing, which 

 would give them full effect. 



It is easily seen that either vane, when turned normal to line of 

 swing, doubles the air resistance of its pendulum as compared with 

 the other pendulum having its vane in the line of swing. 



Prior to adding these very large vanes, the pendulums had been 

 synchronized and their bobs exchanged as in the last entry of Table 

 II., showing the mean time required for the bismuth to gain one beat 

 was 186 minutes. 



When the air resistance of the zinc was thus made double that of 

 the bismuth, the time required for the latter to gain one beat was 

 137 minutes. When these conditions were reversed, giving the bis- 



