478 REPORT— 1886. 



moving and dead load. It has not since been altered, and in practice it is 

 assumed to be applicable to all wrought iron, whether the quality be good 

 or not. 



Among the numerous considerations suggested by the survey (after 

 the lapse of over 25 years) of the period of which a brief sketch has 

 been given, the most striking is the confirmation which experience has 

 given to the conclusions of the Commission of 1847. Derived from the 

 interpretation by skilled mathematicians of the results of experiments 

 conducted by practical engineers, combined with the evidence of the ablest 

 engineers of the time, their conclusions were based upon a solid foundation 

 of fact and experience. 



Their recommendations, although jealously resented by civil engineers 

 at first, notwithstanding the avoidance of legislative interference, by 

 which freedom was secured for the development of engineering science, 

 led, under the judicious interpretation of the inspecting officers, to the 

 present rule for cast iron structures. This rule is good in principle because 

 it derives the load — and consequently the stress — to which the structure 

 may be subjected, from the actual strength of the material. 



The present rule for wrought iron has no such foundation, and it is 

 indeed only due to the high professional attainments of the inspecting 

 officers, and the sound judgment and great moderation with which they 

 dealt with the difficulties which natui-ally arose when wrought iron first 

 became generally used, that the present rule, introduced without special 

 experimental research, has endured so long that it has obtained the 

 sanction of what — to younger engineers at least — is an immemorial usage, 

 taken for granted and stereotyped beyond reach of improvement. 



Its principal fault is in allowing a fixed limit of stress without regard 

 to the quality of the material. This does not lead to serious results so 

 long as the limit of five tons per square inch is understood to represent a 

 factor of safety of four applied to iron having a breaking strength of not less 

 than twenty tons, as explained by Captaiu Tyler in 1859 ; but it must not 

 be forgotten that the rule itself is used by many who do not know its 

 origin, and the absence of any stipulation as to the strength of the mate- 

 rial leads naturally to the assumption that five tons represents the safe 

 working stress for any quality of iron in the market ; and many inex- 

 perienced engineers do so interpret and use it. 



It is hardly necessary to state that there are many qualities of iron for 

 which such an assumption would be attended with considerable danger, 

 but it is not so apparent that a bridge made of utterly untrustworthy 

 material might not, under the ordinary tests, afford any indication of its 

 insecurity. Such is, however, the case, and the safety of the public 

 depends, in this respect, very much more upon the choice of a suitable 

 material by the engineer than upon the Board of Trade rule. 



A fixed limit of stress without regard to the quality of the material 

 also restricts the engineer in the development of economic design in the 

 direction of a greater use of better material, such as angle and bar iron of 

 superior strength and ductility. Nor does a fixed stress offer any induce- 

 ment to the manufacturer to improve the quality of plates. 



These considerations apply with much greater force to the present 

 rule for steel. As a material, steel is much more variable in its strength 

 than iron, which renders the application of an invariable coefficient more 

 objectionable. 



It is true that the Board of Trade, in accordance with a recommenda- 



