OF VULCANIZED INDIA-RUBBER. HEBB. 75 



the kinetic method, the rigidity varied with the angle, and 

 here, also, definite angles of twist were always used. 



In using the static method, the twisting force was applied at 

 the end of the arm carried by the lower brass tube. In the 

 earlier experiments it was applied by means of a thin silk 

 string, horizontal and perpendicjlar to the arm, which passed 

 over the pulley of a set of frictionless wheels taken from an. 

 Attwood's machine, and carried a small plummet of known, 

 weight. The plummet was so light that the cord was not 

 appreciably deflected from the vertical. In order to make the 

 friction as nearly as possible the same in all experiments with 

 the same plummet, I observed the position of the end of the 

 aim before the plummet was attached or the cord twisted, 

 and then having attached the plummet, I determined the 

 amount of twist to be applied in order that the arm might 

 make small oscillations about this position. 



Even with this procedure, however, successive observations 

 showed a lack of agreement which was traceable to friction. 

 Hence, in the later experiments, I used Mallock's* method of 

 applying the force, which I found not only to" give more con- 

 sistent results, but to occupy less time. A small plummet of 

 known weight which was suspended from the end of the arm, 

 by a tine silk string was drawn aside by a second silk string, 

 which was kept horizontal, the two strings being in a plane 

 perpendicular to the arm. The distance to which it was drawn 

 aside was determined by the aid of a second plummet hanging 

 freely from the end of the arm. The horizontal force at the- 

 end of the arm was then equal to the weight of the first 

 plummet multiplied by the ratio of the distance to which it was 

 drawn aside to the distance below the end of the arm of the 

 point of junction of the two strings. These distances, 

 together with the length of the arm, could be measured with 

 consifieiable accuracy, and thus the torque to which the cord 

 was subjected determined. 



Proc. R. S. L., 46, 233, 1889. 



