UNIFICATION OF METHODS IN TESTING MATERIALS. XXXIX, 
tests on timber alone for the United States Government, was 
sufficient guarantee that he was reliable. Professor Johnson in 
dealing with the definition of the French Commissions shewed 
that none of them could be used in practice, and that they were 
absolutely indeterminate or wholly dependent on the delicacy of 
the measuring apparatus rather than on the qualities of the 
material tested. When the most delicate apparatus was employed, 
several specimens from the same bar of the most uniform material 
might give elastic limits of either of the first two kinds which 
differed widely from each other, and hence were materially con- 
tradictory. In other words, such delicate tests were worthless for 
practical purposes, the results obtained not being characteristic. 
The third definition, that of apparent elastic limit was also 
indefinite since it remained a question which load was to be taken, 
the higher load at which the first great permanent elongation 
occurred, or the lower load under which this elongation continued 
to spread throughout the entire length of the bar: these often 
differed by as much as from 1} tons to 3 tons per square inch. 
Prof. Johnson was also seized with the want of knowledge on this 
subject, and stated : “ The fact is something must be done in this 
matter, since no one knows what is meant by elastic limit, without 
an explanation, which explanation is not usually given. 
Prof. Johnson proposed as a way out of the difficulty, the 
adoption, in the future by members of the profession, of an 
arbitrary definition which he styled the “ apparent elastic limit,” 
and defined as being that point on the stress diagram where the 
rate of deformation was 50% greater than it was at the origin. 
This point, in all tension stress diagrams, would be found to mark 
a well defined point, whose coordinates were practically fixed and 
constant for the same material. Although this point was slightly 
beyond the true elastic limit it would mark a point corresponding 
to a permanent set much less than could be measured on any scale 
by the naked eye and hence might be regarded as the true elastic 
limit for commercial purposes. This point also served to cl 
material as to the maximum loads they could resist without receiv- 
