2Q2 



BECKER AND DAY 



plates, the movable scale pan and the fixed bridge, weights 

 could be added in the other pan of the balance which would 

 serve to compress the cylinder, or in the same pan to exert a 

 tensile stress upon it and the change in length be recorded very 

 accurately by the observer at the telescope. A simultaneous 

 measurement of the diameter was obtained b}- mounting 2 

 cameras at right angles to each other and focusing them sharply 

 upon the periphery of the cylinder so as to record 2 perpendicu- 

 lar diameters (Fig. 2). The back of each of these cameras 



Fig. 2. 



was provided with a permanent slit in front of the plate in such 

 a way that the rotation of the plate behind the slit gave succes- 

 sive images of the same portion of the cylinder to the same 

 scale (magnified about 4 times) upon the same negative. 



The operation was then a simple one, requiring 2 observers : 

 As soon as the foam cylinder had been deposited upon the pan, 

 the bridge placed over it and the balance had reached its posi- 

 tion of rest, simultaneous photographs were made with the 2 

 cameras, and at the same moment a reading of the length with 

 the telescope and scale. A small weight was then quickly added 

 in the other pan. This produced a slight compression, the pan 

 came to rest almost immediatel}', whereupon a second reading 

 of the telescope and scale was made and a second pair of photo- 



