424 HEPORT— -1887. 



Report of the Committee, consisting of Mr. W. H. Barlow, Sir F. 

 J. Bramwell, Professor James Thomson, Sir D. Gtalton, Mr. B. 

 Baker, Professor W. C. Unvvin, Professor A. B. W. Kennedy, Mr. 

 C. Barlow, Professor H. S. Hele Shaw, Professor W. C. Roberts- 

 Austen, and Mr. A. T. Atchison (Secretary), appointed for the 

 purpose of obtaining information luith reference to the En- 

 durance of Metals under repeated and varying stresses, and 

 the proper working stresses on Raihvay Bridges and other 

 structures subject to varying loads. 



In a report to the Britisli Association in 1837, on strength and other 

 properties of cast iron, Mr. Eaton Hodgkinson (' Brit. Assoc. Report for 

 1837,' Part i. pages 362 and 363) made announcements, from his experi- 

 mental researches, to the following effect : — That in various experiments 

 on transverse loading of bars he had found visible permanent sets pro 

 duced by such small loadings as ^^-^, -^, and -^^ of the breaking 

 weight ; showing, he said, ' that there is no weight, however small, that 

 will not injure the elasticity ; ' and as a conclusion that ' the maxim of 

 loading hodies within the elastic limit has no foundation in nature. ' 



Again, in the ' Brit. Assoc. Report for 1843,' Part ii. page 24, Mr. 

 Hodgkinson, after detailing further experiments on the same subjects, 

 says : — ' It appears from the experiments that the sets produced in bodies 

 are as the squares of the weights applied, and that there is no weight, 

 however small, that will not produce a set and permanent change in a 

 body, and that bodies when bent have the arrangement of their particles 

 altered to the centre ; and when bodies, as the axles of railway carriages, 

 are alternately bent, first one way and then the opposite, at every revo- 

 lution, we may expect that a total change in the arrangement of their 

 particles will ensue.' 



Such assertions as those in Mr. Hodgkinson's two communications 

 here referred to, if accepted in full, must necessai'ily induce very un- 

 comfortable feelings as to endurance of engineering structures. Mr. 

 (now Professor) James Thomson, however, in a paper published in the 

 ' Cambridge and Dublin Mathematical Journal,' vol. iii. p. 252, Nov, 

 1848, without abandoning the idea of there being some real foundation in 

 nature for prevalent opinions as to limits of elasticity, showed how the 

 elastic range of change of form might, in many of the ordinary cases of 

 materials newly prepared by manufacturing processes, be found to be 

 very narrow on account of the existence of mutual strains or stresses 

 among the particles composing them — that thus permanent sets might be 

 met with on the application of very small loadings — that in this way, 

 through the ductile yielding of the more severely stressed parts, the range 

 of elastic action, or range of action within elastic limits, would be greatly 

 widened, and that after the application of a heavy load, which the material 

 could properly bear, subsequent applications of any smaller loads would 

 produce no new permanent set or alteration — none, at any rate, in any way 

 corresponding to those great and alarming alterations indicated in Mr. 

 Hodgkinson's announcements. (That paper of Professor Thomson's came, 

 besides, under the notice of practical men through its having been re- 



