420 THE SCIENTIFIC PAPERS OF 



that gentleman when he said they should have a harder steel for 

 such purposes as ship-building, and that they should do away with 

 the bending test. 



Mr. Adamson : No, no. 



Dr. Siemens, continuing, said he should strongly advocate the 

 maintenance of that test. Mr. Adamson had alluded to the 

 material for the Frith of Forth Bridge, and he (Dr. Siemens) 

 agreed with him, that where the bars of such a structure as a 

 bridge had to be subjected to a permanent strain, a comparatively 

 hard steel was the most suitable to be employed. The question of 

 the links for that bridge had indeed occupied his attention for 

 some time, and when Sir Thomas Bouch, in the summer of 1878, 

 came to consult him as to their construction, he had an idea of 

 riveting mild plates one-eighth of an inch thick together, but he 

 (Dr. Siemens) strongly advised him to abandon that and to use 

 solid links, making them of a material which would stand a 

 breaking strain of 50 tons, and which they could load with perfect 

 safety to the working strain of 10 tons. While it was perfectly 

 safe to make such a structure as a chain bridge of this material, it 

 would not be safe, and there would be no advantage, he main- 

 tained, in making a ship of it. It might be supposed that the 

 harder material was stifler, and that they might therefore make 

 the plates lighter ; but if they tested a bar of mild steel of a 

 square inch sectional area, and by the side of it a bar of hard steel 

 which would break with a strain of 50 tons to the square inch, 

 they would find that both elongated exactly to the same extent 

 per ton of weight put upon them, up to, say 15 tons ; therefore. 

 it would be wrong to say that the hard steel was, within that 

 limit, more rigid than the mild steel, and this was a fact which he 

 thought was not sufficiently known or borne in mind by engineers. 

 If they continued their tests, increasing the weight beyond 15 tons 

 per square inch, then only a difference became perceptible. The 

 mild steel, when it was strained beyond that point, began to take 

 a permanent set, whereas the hard steel, having a much greater 

 elastic range, would return to its original length until the elastic 

 limit of, say, 25 tons per square inch was reached. 



In selecting a material for ship-building, they should put the 

 problem to themselves in this way : Do we intend to strain this 

 material in any direction beyond 15 tons per square inch ? He 



