XLVI. DISCUSSION. 
he would like to say that the continuation of observation of the 
‘yield point’ was desirable, because although the phenomena of its 
occurrence might not be fully elucidated, they were worthy of 
study, since it marked a critical change in the behaviour of the 
material. In general it was desirable that the results of testing 
should be of value not only from the view of the engineer but also 
from that of the physicist. 
Mr. 8. H. Barractovueu said the discussion had largely centered 
around the question as to what point on the stress diagram should 
be chosen as a measure of the elastic properties of the material 
under test. The whole question was, in the present state of our 
knowledge, almost entirely a matter of personal opinion. He 
could not agree with some of the previous speakers in their 
opposition to the use of the yield point as a satisfactory measure 
of the elastic properties for all “practical” purposes, and he 
thought that in attempting to altogether discredit the worth of 
this point a rather unjustifiable use had been made of Professor 
Johnston’s lately published book, for on page 306 of that work it 
was stated that “in practical or commercial testing it will be 
found sufficient to observe the third one only. . .” i.¢., the yield 
point. Further the fact that the stress-strains diagram obtained 
from a test of certain materials such as cast iron and some grades 
of steel did not show any yield point, was no argument against 
its use for such materials as did show it. An examination of the 
results of tests made at the University for some years past would 
show that in the great majority of cases the yield point is well 
marked, and constitutes a thoroughly characteristic point on the 
diagram. The yield point is of course considerably above the 
elastic limit, and, as the author of the paper states, should always 
be called the yield point and not the elastic limit, when reporting 
atest. To illustrate the positions of these two points and of the 
two arbitrary elastic limits which had been proposed during the 
course of the discussion, the speaker had prepared two stress-strains 
diagrams representing the results of tests made with the aid of 
Marten’s mirror extensometer, by which the extensions of the test 
