474 SCIENCE PROGRESS 



stress in the ideal case when the stress is only applied for an 

 infinitely short space of time. It has been pointed out earlier 

 in this article that under these circumstances the recovery from 

 strain after release is at once complete. The wire behaves just 

 as it would do under similar conditions for stresses less than the 

 elastic limit. It seems reasonable to suppose, therefore, that the 

 curve ra will simply be a prolongation of the straight line 

 o r, which represents strain against stress in the truly elastic 

 region. The curve r b will give the value of the strain for any 

 particular value of the stress, at a time one minute after the 

 application of the latter, and in all cases this value will be 

 greater than that corresponding to a stress of momentary 

 duration. Similarly the curve rc indicates the various values 

 of the strain after two-minute intervals. A diagram founded on 

 experiment, in order to be complete, would contain many more 

 curves than those indicated, and would probably consist of 

 curved lines, as there is no adequate reason for supposing that 

 they would be straight. The vertical straight line r i represents 

 a limit beyond which these curves cannot be. If a force were 

 applied to the end of a wire and continued to act for an infinite 

 time, the strain could not become more than infinitely large, and 

 would probably not do so. If we denote by T the time during 

 which the stress is in action, then the curve for T = o is ra, 

 and that for T = co a curve somewhere to the right of r i ; and 

 between these two all the other curves, representing various 

 values of T, must lie. A. diagram consisting of a series of curves 

 such as those just described would certainly be very interesting 

 and instructive, and doubtless also very useful. 



Several attempts have been made to develop a theory which 

 will produce results consistent with those which have been 

 observed, but none has been uniformly successful. The difficul- 

 ties in the way of such an attempt are certainly very great. 

 The facts observed cannot be accounted for by internal friction 

 in the ordinary sense of the term. The bodies which exhibit 

 this elastic after-effect certainly appear to possess elastic and 

 viscous properties ; but these properties are not independent, 

 and cannot be treated as such. Besides this, there is the fact 

 brought to light by Kohlrausch's experiment described above, 

 that a body is capable of responding to more than one over-strain 

 at one and the same time : in other words, that the strain in 

 a body is not only dependent on the stress at the moment in 



