COXSTITtTIOX OF BODIES. 



week and lie al*> found that if the wire was set in vibration the motion of 

 the point .f equilibrium was more rapid than when the wire was not in vibration. 



We mav produce a very complicated series of motions of the lower end 

 of tbe win by previously subjecting the wire to a series of twists. For in- 

 rtanf*. we KAY frst twist it in the positive direction, and keep it twisted for 

 a < j -V( then in tbe negative direction for an hour, and then in the positive 

 direction for a minute. When the wire is left to itself the displacement, at 

 fin* positive, becomes negative in a few seconds, and this negative displacement 

 increases for some time. It then diminishes, and the displacement becomes 

 utive, and lasts a longer time, till it too finally dies away. 



Tbe phenomena are in some respects analogous to the variations of the 

 surface temperature of a very large ball of iron which has been heated in a 

 furnace for a day, then placed in melting ice for an hour, then in boiling 

 water for a minute, and then exposed to the air ; but a still more perfect 

 analogy may be found in the variations of potential of a Leyden jar which 

 bat been charged positively for a day, negatively for an hour, and positively 

 again for a minute*. 



The effects of successive magnetization on iron and steel are also in many 

 respects analogous to those of strain and electrification f. 



The method proposed by Boltzmann for representing such phenomena mathe- 

 matically is to express the actual stress, L (f) , in terms not only of the actual 

 Htruin, (l , but of the strains to which the body has been subjected during 

 all previous time. 



His equation is of the form 



-f 



Jo 



where <a is the interval of time reckoned backwards from the actual time t to 

 the time t-u, when the strain O t _ m existed, and $ (o>) is some t function of 

 that intcrv.il. 



\Ve may describe this method of deducing the actual state from the pre- 



us states as the historical method, because it involves a knowledge of the 



previous history of the body. But this method may be transformed into another, 



1 tT ^ "P kin8on ' ' <0n the R^l Charge of the Loyden Jar," Proc. R. S. iv. 408, 

 .Mmreb 30, 18 1 '. 



t See Wicdctnnn' Galvanitmtu, vol. n. p. 567. 



