270 THE SCIENTIFIC PAPERS OF 



those of Hodgkinson and Fairbairn. Possibly it might be the 

 case that, in subjecting a bar for the first time to a strain, elonga- 

 tion did take place, which might be explained in this way : it was 

 conceded that in the manufacture of a bar of iron internal strains 

 must arise, owing to the different action of the rollers upon 

 different portions of the material, and such " virgin " bar might 

 be regarded as consisting of a number of aggregate fibres of metal 

 strained one against the other, the internal metal, which had been 

 the hottest, being permanently in tension, and the external metal 

 in compression. In subjecting such a bar to a moderate strain, 

 only the internal fibres would be permanently elongated, while 

 the external metal would be simply released from compressive 

 strain, after which all parts of the bar would bear the strain 

 equally, and this equalising of the active forces must necessarily 

 be accompanied by a moderate stretch. Therefore, in order to 

 ascertain the true limit of elasticity of a material, the first experi- 

 ment upon a virgin bar should not be taken, but the strain should 

 be continued till it had reached a certain moderate limit, when it 

 should be reduced, and then the same series should be gone 

 through again. In proceeding thus he thought it would be found 

 that there was no permanent elongation till the true limit of 

 elasticity was reached. This true limit of elasticity he regarded 

 as being a very important point. Mr. Phipps had justly remarked 

 that, in the long series of experiments on breaking strains, 

 published by various authors, a conclusion was arrived at, more by 

 guess than by scientific reasoning, to the effect that a load might 

 be applied to, say, one-fourth of the breaking strain, with tolerable 

 safety, without any distinction being made regarding the limit of 

 elasticity of the particular material under examination. It would 

 be found, however, that two bars of metal, which might break at 

 nearly the same point, and which might consist, chemically 

 speaking, of nearly the same materials, would be capable of 

 bearing with safety a very different load. The one might begin 

 to elongate with less than one-half the breaking strain, and the 

 other would not begin to elongate till two-thirds of the breaking 

 strain had been reached. He had made some experiments with 

 cast steel, which, being the most refined and uniform metal, was 

 best adapted for such experiments, proving these discrepancies 

 according to the mode of its production ; and he had no doubt 



