TRANSACTIONS OF SECTION G. — PBESIDENTIAL ADDRESS. 657 



SectiOxV G.— engineering. 

 President of the Section — J. A. Ewing, M.A., LL.D., F.R.S. 



THURSDAY, AUGUST 2. 

 The President delivered the following Address : — 



I INTEND to devote this Address to considering in certain aspects the inner 

 structure of metals and the manner in which they yield under strain. It will not 

 be disputed that this is a primary concern of the engineer, who in all his problems 

 of design is confronted by the limitations imposed on him by the strength and 

 elasticity of the materials he employs. It is a leading aim with him to secure 

 lightness and cheapness by giving to the parts such dimensions as are no larger 

 than will secure safety, and hence it is of the first importance to know in each 

 particular case how high a stress may be applied without risk of rupture or of per- 

 manent alteration in form. Again, the engineer recognises the merit, for structural 

 purposes, of plasticity as well as strength, and in many of his operations he makes 

 direct use of that property, as in the drawing of wires and tubes or the flanging of 

 plates. He is concerned, too, with the hardening effect that occurs in such processes 

 when work is expended on permanently deforming a metal in the cold state, and also 

 with the restoration to the normal condition of comparative softness which can be 

 brought about by annealing. Nor can he afford to be indifferent to the phenomena 

 of ' fatigue ' in metals, which manifest themselves when a piece is subjected to 

 repeated alternation or variations of stress — fatigue of strength and fatigue of 

 elasticity, which, like physiological fatigue, admits under some conditions of rest- 

 cure, inasmuch as it tends to disappear with the lapse of time. No apology need 

 be made in selecting for a Presidential Address to Section G a subject that 

 touches so many points of direct practical interest to engineers. It is a subject 

 which has for me the additional attraction of lying in the borderland between 

 engineering and physics — a borderland in which I have often strayed, and still 

 love to stray, and I enter it to-day even at the risk of wandering into regions 

 which, to engineers, may seem a little remote from home, regions where the land- 

 scape has, perhaps, a suspicious likeness to that of the country over which the 

 learned men of Section A hold rule. 



To engineers, quite as much as to physicists and chemists, we owe in recent 

 years an immense extension of knowledge regarding the structure of metals. This 

 has come about mainly by the intelligent use of the microscope. Take any piece 

 of metal, in the state in which an engineer makes use of it, polish and lightly etch its 

 surface, and examine it under the microscope, and you find that it is a congeries of a 

 multitude of grains, every one of which may be proved to be a ciystal. It is true 

 that the boundaries of each grain have none of the characteristics of geometrical 

 regularity which one is apt to look for in a crystal, but the grain is a true crystal 

 for all that. Its boundaries have been determined by the accident of its growth in 

 relation to the simultaneous growth of neighbouring grains — the grains have 

 grown, crystal fashion, until they have met, and the surface of meeting, whatever 



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