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. If 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. 



