S/K WILLIAM SIEMENS, /-.A'..s. 



397 



but \\lien 24 tons were applied, then a permanent set 

 occurred also in the hard bars ; therefore the elastic limit of the 

 hard steel was 24 tons, whereas that of the milder steel was only 

 18 tons. 



r.nt there is a very peculiar circumstance connected with these 

 elongations. If a bar of mild steel is taken from the rolls and 

 subjected at once to a test of say 18 tons, it will very likely be 

 found to show a permanent elongation, but if the same bar is first 

 subjected to a strain of say 17 tons, for several hours, it will then 

 be capable of resisting perhaps 19 or 20 tons before showing any 

 permanent elongation. One might also say the bar of steel can be 

 taught to resist a higher strain without yielding permanently. Sir 

 "William Thomson has lately made some elaborate researches on 

 this point, and perhaps he will favour the Meeting with some 

 account of them. 



The question of using steel for the purposes of engineering or 

 military construction depends a great deal upon the particular 

 application. Mild steel has the peculiar quality of yielding to an 

 enormous extent to strain before giving signs of rupture. Bars 

 are tested generally to 28 tons, at which mild steel, such as is used 

 in naval construction, generally breaks after showing an elongation 

 of 25 per cent. The steel used in the construction of boilers, 

 which is made still milder, will stand only a total strain of 24 tons, 

 but will show a still greater elongation before breaking. From 

 this we can go upwards, and produce steel that will bear a break- 

 ing strain of 50 tons, with an elongation of perhaps 12. per cent. 

 Still harder steel shows a strength of GO tons, and an elongation of 

 only 7 or 8 per cent, before breaking, whilst Sir Joseph Whit- 

 worth has shown that the absolute strength of steel in the form of 

 bars, of the proper temper, may be brought up to 90 tons per 

 square inch, by subjecting it to a process of oil hardening ; such a 

 tensile strength is hardly exceeded by carefully tempered steel 

 wire. Therefore we have a range of strength which we can apply 

 under different circumstances with great advantage. It must be 

 borne in mind that the harder steel is apt to become brittle when 

 suddenly cooled, and therein consists the great safety of using the 

 milder description of steel for engineering and military purposes. 

 With regard to this last matter I would say something before con- 

 cluding, with reference to the construction of ordnance. 



