G.— ENGINEERING 171 



reason that though engineering is not a separate branch of natural philo- 

 sophy, but natural philosophy studied with a view to application, yet the 

 attitude of the engineer to his problems is as I believe something both 

 peculiar and worth preserving. 



It will suffice to explain my meaning if I make comparisons with the 

 mathematician and physicist, leaving others better qualified to deal in 

 like manner with the chemists. Wherein, then, does the outlook of the 

 engineer differ from that of the physicist ? Mainly, I think, in that his 

 problems are inexorable, and he recognises them as such. The physicist 

 despairing of progress along a path attempted, is free to try some 

 other : the engineer has to solve the problem as it is presented, and some 

 solution he must have, even though it be only approximate. It has been 

 the fashion of late to jeer at the engineer's ' factor of safety ' — changing 

 its name to ' factor of ignorance ', and asserting that like charity it covers 

 a multitude of sins. We must I think admit the criticism to be largely 

 true as regards the past : too often factors of safety have been a refuge 

 and an excuse rather than the extra assurance that they ought to be. But 

 they have come down greatly of late, since aeronautics set an added value 

 on weight-saving achieved without loss of efficiency ; and the time I think 

 is near when they will have values strictly dependent on the reliability 

 of our materials. As ' factors of uncertainty ' they will always have a 

 raison d'etre. 



Now uncertainty of this kind does not, as I see the matter, enter into 

 the physicist's scheme of things at all. (He has his own ' uncertainty 

 principle ' — so quaintly advanced in recent years as an argument for human 

 free will ; but I can conceive no argument for free will based upon the 

 variability of constructional materials, — the Victorians missed no path to 

 spiritual comfort there !) The physicist's problems are fundamental, 

 and he is not the man to let them be complicated by additional difficulties. 

 If corrosion is a potential source of trouble, then he will use gold if need 

 be ; if magnetic flux is calculable only for one or two particular shapes, 

 then he will use those shapes. Because throughout he is free to choose ; 

 his shapes are not dictated by constructional or manufacturing require- 

 ments, nor his materials by considerations of strength or cost. 



Simple illustrations are best : let us visualise the attitude to elasticity 

 of a physicist who still retains some interest in nineteenth-century physics. 

 He will be interested in Hooke's law, and in its interpretation as a statistical 

 average of effects due to forces from very many atoms. He will recognise 

 two distinct types of strain, the first involving change of dimensions 

 without change of shape, the second change of shape without change of 

 volume ; and he will devise ingenious experiments for measuring the 

 two relevant elastic moduli. In this connection he will study Saint- 

 Venant's theories of torsion and of flexure, and he may even pursue the 

 harder parts of elastic theory with the aim of eliminating errors in measure- 

 ment that result from straining due to weight. But speak to him of the 

 strength and distortion of an engine crankshaft — a matter of interest in 

 practice, so long as engines tend to fail by torsional vibration ; and if you 

 find him interested then — well, he is an engineer in disguise ! For 

 speaking qua physicist he will say : * I see that both torsion and flexure 

 are involved — that is, both of the two fundamental types of strain ; but 



