120 



HENRY A. ROWLAND 



markable; but still we see a little tendency in the end observation to 

 rise above the value given by the formula. 

 In equation (7), and also from Green's formula, we have seen that 



* T 



for a given quality and temper of steel p = r - is a constant. From 



to 



Coulomb's experiments on a steel bar -176 inch in diameter (whose 

 quality and temper is unknown, though it was probably hardened) Green 

 has calculated the value of this constant, and obtained -05482, which 

 was found from the French inch as the unit of length, but which is 

 constant for all systems. From Tables XIII and XIV we find the value 



TABLE XIII. 



TABLE XIV. 



of r to be -4674, whence ^= -04440 for steel not hardened. As the 



steel becomes harder this quantity increases, and can probably reach 

 about twice this for very hard steel. 



To show the effect of hardening. I broke the bar used in Table XIV 

 at the centre, thus producing two bars 6-4 inches long. One of these 

 halves was hardened till it could scarcely be scratched by a file ; but the 

 other half was left unaltered. The following Table gives the distribu- 

 tion, using the same unit as that of Tables XIII and XIV. The bars 

 were so short that the results can hardly be relied on ; but they will at 

 least suffice to show the change. 



