1889.] Properties of Vulcanised India-rubber, 



239 



Then w = a — - — s, 



and since india-rubber is nearly incompressible, Is = l's', 



Vw 



hence q = -r v — • 



x s(l—l) 



To show the sort of agreement among themselves of the measures 

 made in this way, I subjoin a table showing the results of five 

 experiments, chosen at random from many others, on each of the 

 kinds of india-rubber used, the units being inches and pounds. 



Soft 



grey. 



Bed. 



Hard 



grey. 



I = 10. s = 0-2690. 



1 = 10. 5 



= 0-2307. 



I = 10. & 



= 0-2625. 



V-l. 



2- 



V-V 



1- 



V-l. 



2- 



0-228 



124 9 







0-163 



161 -1 



0-038 



497 -0 



0-385 



125 -3 



0-345 



163 -0 



0-078 



491-2 



0-530 



129 '2 



-471 



166 -9 1 



0-115 



502-3 



1-77 



123-9 



1-430 



166-5 



0-156 



495-7 



4-85 



114 -0 



3-390 



164-5 



0-360 



463 -5 



The lowest values for q are those given by experiments in which 

 the stretching force acted for the longest time. 



There is evidence also, which appears more strongly in the results 

 represented by Diagram III, that q diminishes with the extension 

 until the stretched length is about 3/2 times the natural length. 



(2.) Young's Modulus. Dynamical Measure. 



AP (fig. 3) is a pendulum. The strip of india-rubber DC was held 

 rigidly at D, and attached at to the arm AB bracketed out from the 

 pendulum. 



The experiments were made by observing the period of the 

 pendulum with the india-rubber attached, and noting the difference 

 between this and the natural period of the pendulum. 



The india-rubber was, of course, initially strained a little, and the 

 amplitude of the vibrations used was never great enough to make the 

 strain vanish. 



Let T be the natural period of the pendulum, 



\ = length of equivalent simple pendulum, 

 T x = period of pendulum with india-rubber attached, 



VOL. XLVI. 



s 



