CONSTITUTION OP BODIES. 621 



but on the previous history of the body. Thus the stress is somewhat greater 

 when the strain is increasing than when it is diminishing, and if the strain 

 is continued for a long time, the body, when left to itself, does not at once 

 return to its original shape, but appears to have taken a set, which, however, 

 is not a permanent set, for the body slowly creeps back towards its original 

 shape with a motion which may be observed to go on for hours and even 

 weeks after the body is left to itself. 



Phenomena of this kind were pointed out by Weber and Kohlrausch (Pogy. 

 Ann. Bd. 54, 119 and 128), and have been described by O. E. Meyer (Poyy. 

 Ann. Bd. 131, 108), and by Maxwell (Phil. Trans. 1866, p. 249), and a theory 

 of the phenomena has been proposed by Dr L. Boltzmann (Wiener Sitzunyx- 

 berichte, 8th October 1874). 



The German writers refer to the phenomena by the name of "elastische 

 Nachwirkung," which might be translated "elastic reaction" if the word reaction 

 were not already used in a different sense. Sir W. Thomson speaks of the 

 viscosity of elastic bodies. 



The phenomena are most easily observed by twisting a fine wire suspended 

 from a fixed support, and having a small mirror suspended from the lower end, 

 the position of which can be observed in the usual way by means of a tele- 

 scope and scale. If the lower end of the wire is turned round through an 

 angle not too great, and then left to itself, the mirror makes oscillations, the 

 extent of which may be read off on the scale. These oscillations decay much 

 more rapidly than if the only retarding force were the resistance of the air, 

 shewing that the force of torsion in the wire must be greater when the twist 

 is increasing than when it is diminishing. This is the phenomenon described 

 by Sir W. Thomson under the name of the viscosity of elastic solids. But 

 we may also ascertain the middle point of these oscillations, or the point of 

 temporary equilibrium when the oscillations have subsided, and trace the varia- 

 tions of its position. 



If we begin by keeping the wire twisted, say for a minute or an hour, 

 and then leave it to itself, we find that the point of temporary equilibrium is 

 displaced in the direction of twisting, and that this displacement is greater the 

 longer the wire has been kept twisted. But this displacement of the point of 

 equilibrium is not of the nature of a permanent set, for the wire, if left to 

 itself, creeps back towards its original position, but always slower and slower. 

 This slow motion has been observed by the writer going on for more than a 



