302 DR. L. N. G. FILON ON THE 



Assuming the law connecting C and X to be given by (14), we have 



C- r 1 <*>//! \ w/i^ r" P w//i \ < 2 >/x'\ 

 j \JQ f{l Ay /A^, \j j V>o /\A A u ^A ^, 



where G^, V 2> refer to the slab F. 

 Hence 



Therefore 



and from equation (5), 



/I 



H,(X'-X)/C (a> = -3M a o-A/t/26/. 

 Co (2) =-2n(X'-X)&//3M J o-A& (32), 



>A (33), 



Co (1) = 2n (X'-X) &! (I + 1 

 \<r 



which give the absolute values of C for both slabs. 



A great many errors enter into the determination of these absolute values. It is 

 very difficult to measure the spans with sufficient exactness, and the differences X' X 

 are not large enough to allow of very accurate determination. 



23. Effect of Chemical Composition on Stress-optical Properties. 



The mean values for C rt obtained in this way for each beam are shown below in 

 Table VI. A, B denote the two individual slabs of each pair, and C is expressed in 

 a unit equal to 10~ 7 (cm.) 2 per kilogramme weight. 



TABLE VI. Dependence of C , X,, on Chemical Composition. 



* 1-93 has been taken, and not the mean of the two values, because here C u certainly differs for slabs 

 A and B, and A was the slab analysed. 



t Estimated from the values of A r on p. 291. 



