The Strength of Materials as Applied to Engineering. 155 



these experiments are to be trusted, the contrary appears to be the case, 

 and that very heavy strains, even to the extent of the breaking strain, 

 and this even several times repeated, actually increases the strength of 

 iron by every application. The result of several experiments are stated 

 as follows on ' Iron ' : 



*' ist fracture, mean breaking strain of 33}4 tons; 2d fracture, 35^ tons; 

 3d fracture, 37)4^ tons ; 4th fracture, 4Q\ tons ; diff. Q\\ 20 per cent. 

 •^4"' These facts,' says the official report, 'appeared so important, and 

 were so much opposed to the opinion of the best informed persons upon 

 the subject, that in order to prove beyond all question that the increased 

 strength was due to the repealed previous strains, and not to the dimin- 

 ished length of the bar, we considered it proper to make experiments 

 upon this point. 



" ' We therefore took several bars of the same kind of iron, of the same 

 diameter, but varying in length from ten inches to ten feet, and found 

 they were all equally strong. It may be proper to state the circum- 

 stances which usually attend the fracture of iron bars, namely, that con- 

 siderable heat is evolved at the breaking place,^nd that the diameter of 

 the bar is everywhere reduced, but particularly so at the place of frac- 

 ture ; but it is worthy of remark that at the second and subsequent frac- 

 tures of the bars, it generally happened that little or no heat was given 

 out at the place of fracture, and that the general diameter of the bar was 

 not again sensibly reduced, except in some instances, at the place of 

 fracture.' The publication of these results in a government blue book is 

 sure to attract general attention to this important question." 



I sbould say it would. 



All experimenters, without any exception, men who have 

 made this problem their particular study report results in contra- 

 diction to the foregoing article. In reading the above, the ques- 

 tion would occur to some. Who would think of testing a bar 

 after the same was ooce broken or injured? But it seems that 

 experimenters did think of it, and they did also faithfully re- 

 port the results, and turthermore, they all agreed exactly upon 

 this point, more so than upon many others. Their unilorm re- 

 ported results were : that the strain exceeding the limits of 

 elasticity and producing permanent elongation, could not be 

 borne for any considerable length of time. Yet it might be 

 argued tbat the relaxation of the strain would have had some 

 influence on the results. This, however, was also thought of by 



