TRAVELING AT HIGH SPEEDS—HELE-SHAW. 649 
before the Institution, and he asked the members to read the-explanation, given in the 
clearest possible language, as to why a specimen which was subjected to repeated 
reversal of stress must, according to the teaching of microphotography, inevitably fail 
at a lower stress than under the ordinary test. A certain number of crystals always 
occupied an unfavorable position, and they yielded at a certain stress before the other 
particles yielded. Ii they were allowed to remain in that new position they might 
possibly fix themselves there, but if the stress was reversed the movement found these 
particles in a weakened state; and if it was again reversed they again gave way, and 
thus threw a greater stress on the surrounding particles, and at last caused fissures to 
form and a fracture to take place. This was the ultimate cause of the apparent crys- 
tallization of a fractured specimen. Engineers used to be taught, only a little while 
ago, that a metal crystallized after a certain time when it was subjected to alternating 
stresses. A direct contradiction of that teaching was contained in Dr. Rosenhain’s 
lectures. As a matter of fact it was not a crystallization. The crystals were there 
before, and they were there afterwards. The facets which had a crystallized appear- 
ance were facets of fracture which were gradually produced by the alternation of 
stresses, and thereby the breaking down of the specimen was obtained. He thought 
the members would agree with him that one thing was very apparent from the paper 
and the discussion, namely, the limited extent of our knowledge of the subject. After 
listening to the remarks of the authors and of the gentlemen who had taken part in the 
discussion, the members must feel they had a great way yet to go before the real nature 
of the many phenomena of the resistance of metals was understood. He trusted that 
more experiments would be carried out, with the object of teaching the members 
to realize what was actually going on in the materials they had to use, and at the same 
time he desired to thank the authors for their extremely valuable experimental work, 
which was of a character to aid the building up of a sound theory of the subject. 
