238 



Mr. A. Mallock. The Physical [June 6, 



a cord passing over a pulley, by means of which it can be subjected 

 to any desired strain. 



Two very fine pins P, P' (fig. 2), were fixed in the india-rubber at a 

 distance of 10 inches from one another, and a thin strip of ebonite, E, 



Fig. 2. 



having near one end a hole the^size of the pin, and a mark, M,',10 inches 

 from the hole, was then placed against the india-rubber with the pin 

 P' passing through the hole ; thus, when unstrained, the pin P was 

 exactly on the same level as M ; when the india-rubber was strained 

 the extension PM was measured by the cathetometer T — 



Let w = stretching force, 



I = unstrained distance between P and P', 



V = distance between P and P' under the action of w, 



s = sectional area when length is 7, 



S = ;> jj ,5 ^ ) 



q = Young's modal us. 



