278 



ON THE RIGIDITY 



TABLE I. 



Although the above table shows considerable disagreement 

 "between successive observations, due probably in some measure 

 to friction, to difference of temperature slightly, and to 

 inequality in the times of application of the loads, the values 

 obtained clearly increase with the tension, and give no indication 

 of a minimum point. This result is in agreement with Mr. Mac- 

 donald's last series of observations, and seems to show that the 

 rigidity increases steadily with tension in a cord which has 

 been subjected to sufficiently prolonged extension, provided at 

 least a day is allowed to intervene between increasing the ten- 

 sion and determining the rigidity. 



The next observations were made on a fresh cord of a 

 different rubber from Mr. Macdonald's. It was harder, and 

 contained 60 per cent of pure rubber, according to the state- 

 ment of the manufacturer. A freshly-cut surface showed a 

 dark gray colour. 



All the series of observations given below were made on 

 rubber cords of this kind. 



Both static and kinetic methods were applied, very little 

 time being allowed to intervene between the two determina- 

 tions, in order that the cord might be in the same state, as 



