the Limit of Elasticity in Metals. 201 



is not stated. But doubtless neither rolling nor forging could 

 produce in these bars a stretching at all comparable with that 

 produced by drawing the wire. The limit of elasticity is there- 

 fore here quickly attained. As already remarked, it is not 

 necessary with Morin to dispute entirely the permanent elonga- 

 tions observed by Hodgkinson. 



Far from favouring one view, and rejecting the other, we 

 may assume that both Morin and Hodgkinson have reason for 

 their views — the former as regards the high position of the 

 limit of elasticity in wires, the latter as regards its low position 

 in the case of bars. The difference in the results, it must there- 

 fore be assumed, lies in the different treatment which the ma- 

 terial of the two observers had previously undergone. 



§ 7. On the Permanent Elongations on repeated stretching with 

 the same Load. 



In the foregoing it has been shown that the permanent elon- 

 gations continually increase with increasing loads. But even 

 when the load is several times successively the same, each time 

 an increased permanent elongation is produced. The law for 

 the relation between these elongations is certainly dependent on 

 the position of the point of the curve at which the experiment 

 is made ; yet the following data, which are the means of a num- 

 ber of such cases observed, may render evident the phenomenon 

 in question. If the following increments of the elongation be 

 compared with that which the bar underwent when it was 

 stretched for the first time with the given load, the following 

 values are obtained : — 









Table IV. 





No. 



1. 



Permanent elongation . 



. 1-00 



a 



2. 



}} 



a 



. 0-55 



}j 



3. 



)i 



» • 



. 0-37 



)) 



4. 



jy 



)> 



. 028 



}> 



5. 



33 



>) 



. 0-25 



a 



6. 



)) 



i) • 



. 0-18 



In fig. 4, Plate III., the ordinates represent the order of the 

 numbers of the stretchings, and the abscissae the corresponding 

 total elongations, thus 1*00, 1*55, 1*92, &c. 



It is true the new permanent elongation which is here each 

 time obtained is always smaller and smaller, and it would con- 

 sequently be supposed that after a great number of loads no new 

 permanent elongation would be observed. But even if such a 

 limit could be attained, it is probable that after the lapse of a 

 long time a new permanent elongation would arise notwith- 



