714 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



delicate method of determining whether a casting which is hard on the 

 surface is also hard throughout the interior. The two halves of each of 

 two thick castings of the shape 4, weighing about 4800 grams each, were 

 wound as uniformly as we well could with 156 turns of insulated wire, 



Figure 8. 



and the two coils of each casting were so connected in series that when 

 a current was sent through them both conspired to make one of the pro- 

 jections (say X) a north pole and the other (Y) a south pole: one of the 

 castings was soft while the other, as another experiment afterwards 

 proved, had been imperfectly hardened. With each of these castings a 

 rough kind of hysteresis diagram was obtained by measuring for diffeieut 



