MILD STEEL. 51 



The " elastic stress " or the " ekistic limit" of iron and 

 steel has never, as far as I can ascertain, been very satis- 

 factorily determined, probably from the fact that it has 

 never been defined what particular property the term actu- 

 ally signifies. 



Mr. Kirkcaldy defines the "elastic stress" to be what I 



may call the stress at which the material changes from the 



"passive" to the "active state of distress"; and this is 



illustrated by the results of some tests of mild steel, in 



which he found that after a tensile stress of 5 tons per 



sq. inch had been applied, the extension increased by equal 



increments of length for every 2,000 lbs. increment of stress 



up to 16 tons. At this point the increment of extension 



suddenly increased in a much more rapid ratio per increment 



of stress up to the ultimate stress, showing that the material 



has at that point passed its normal state of elasticity, and 



that at 90 % of the ultimate stress the extension was only § 



of the ultimate extension, the remaining one-third of the 



extension being accomplished during the application of the 



last 10% of the ultimate stress. This point appears to me 



to fix the position of the " elastic limit " more satisfactorily 



than any definition that has been previously suggested. 



Until a few years ago I think I may venture to say that 

 Avrought iron held the first place amongst constructive 

 materials; it was the engineer's best friend, upon it he 

 relied with considerable confidence in the production of his 

 greatest works. It has sustained that confidence well, not- 

 withstanding its many defects, for which I fear that "arch 

 enemy " to perfection, the " greed of gain," is mainly re- 

 sponsible ; but the advance of science, not now content with 

 the old rate of progress, but forging ahead with leaps and 

 bounds, has brought into use the mild steel of which I have 

 been speaking, and to which I think we must all agree to 



