XXXVIII. DISCUSSION. 
elasticity was reached, and where permanent deformation com- 
menced. Just to illustrate the differences of opinion which existed 
amongst engineers as to the elastic limit, he would bring under 
the notice of members the following facts corroborating his 
opinion on that point. The French Commission made use of 
three terms :— 
(1) The elastic limit or the unit stress beyond -which a portion 
of the deformation remains as permanent set. 
(2) The proportional elastic limit, corresponding to the point 
where the deformation ceases to be proportional to the loads. 
(3) The apparent elastic limit corresponding to the point where 
the deformations increase rapidly without any increase in 
the force exerted: all these points lay between ¢ and y of 
the author’s diagrams. 
The Conventions required that during the elastic period there 
should be sought the yield point and the limit of proportional 
elongation, appearing to admit that these two limits were blended. 
The American Society required the determination of the same 
information, insisting especially upon the importance of the yield 
point, which measurement it claimed should be made with the 
greatest precision. The American Society did not however seem 
to be very sure as to their actual requirements, as it defined the 
yield point as the load which produced a modification in the rate 
of elongation, thus seeming to identify it with the proportional 
limit of the French Commission ; but further on in the American 
Society’s report, in an illustration, it required that the limit (yield 
point) should be determined by noting the point at which the 
elongation was suddenly augmented, thus identifying it with the 
apparent limit of the French Commission. 
Prof. J. B. Johnson of Washington University in his recent 
book on ‘ Materials of Construction,” went very fully into this 
matter with a view to put an end to these differences amongst 
professional men. The Professor referred to was a recognised 
authority, and the very fact that he had made upwards of 40,000 
