460 THE SCIENTIFIC PAPERS OF 



one-eighth of its breaking strain, whereas, in the case of wrought- 

 iron, it was thought safe to go to one-fourth. Why was this the 

 case ? Because wrought iron would elongate before it broke ; but 

 in dealing with mild steel, an elongation to more than double that 

 in the case of wrought-iron took place before fracture, and there- 

 fore the same reason that would make an engineer trust wrought- 

 iron to a higher proportion of its ultimate strength than cast-iron, 

 would lead him to trust steel to a still higher limit as compared 

 with its own breaking strain. There was another reason why mild 

 steel could be trusted to a limit exceeding half the elastic limit, 

 namely, that in riveting or punching, its strength was not dimin- 

 ished in anything like the same extent as in the case of wrought 

 iron or of harder steel. He had caused a great many experi- 

 ments to be made with mild steel, and had found that the strength 

 per square inch of the remaining section of material, after a line 

 of holes had been punched in it, was nearly equal, and in some 

 cases, even superior, to the original strength of the bar per square 

 inch of section. That seemed strange but it was perfectly con- 

 sistent. Taking a bar of this material, and putting it under 

 tensile strain nearly equal to the elastic limit, for a length of time, 

 it would be found that not only was the elastic limit raised, but 

 that the ultimate strength was raised, and that very materially. 

 It was possible to teach a bar to carry at least 10 per cent, more 

 than it did on the first trial, by simply keeping it under strain for 

 a length of time. In the same way in subjecting the material to 

 a kind of solid flow any kind of strain its ultimate strength 

 and its elastic limit would be raised ; and the reason why punching 

 or ill-treatment of any kind did not injure such material was that 

 it obeyed the law of solid flow ; whereas, an inferior material, 

 would not in its entirety, obey that law, but would be liable to 

 rupture in its ultimate parts. He, therefore, submitted, that in 

 using steel for riveted structures only the mild kind should be 

 employed. It might be said that in that case it would be neces- 

 sary to give up the strength of the material and approach the 

 ultimate strength of iron ; and what was the use of the material 

 if no more could be done than that with it ? His answer would 

 be that it was a safer material when put together than the stronger 

 steel, and much safer than iron (although its ultimate strength 

 was not much greater,) for the simple reason that there was a 



