OF VULCANIZED INDIA-RUBBER —HEBB, Party 
expressed as in the previous formula, J is moment of inertia 
expressed in lbs. and inches, and ¢ is the time of a complete 
oscillation expressed in seconds. The moment of inertia of the 
plates used in stretching the cords was found by means of the 
following formula: J = Af (a? — b?) /12,in which M is mass of 
plates in Ibs. and @ and 6 are the lengths of sides of plates in 
inches. ‘The moment of inertia of the brass tube at the end of 
the cord was found to be negligible. 
The values of the rigidity determined as above would thus 
be expressed in inch-lb-second gravitational units. 
The effects on the calculated values of the rigidity, of the 
above possible errors of the component observations, were 
calculated in a few cases, and were found in the static observa- 
tions to be between 1.57% and 4.5%, and in the case of the 
kinetic observations to be between 2.5/ and 37% 
My first observations were made on the cord which Mr. 
Macdonald had previously used. He had subjected it to repeated 
and prolonged extension, and found that the minimum point 
which in the earlier series of observations seemed to characterize 
the variation of its rigidity with tension finally disappeared. 
Since the time of his experiments it had lain for six months 
unstretched. 
The static method was the only one employed, the torque 
being applied by means of the frictionless wheels. The proce- 
dure was as follows: First a weight was hung on the cord, and 
left there for at least a day. Then its rividity was determined, 
after which the load was increased and the cord left 
for another day. On the third day the same process was 
repeated, and so on. It was not until later on in the 
experiments that the importance of allowing the loads to act 
for corresponding intervals of time was realized. The following 
table gives the results obtained : 
