Ki'.iiUTY OF ROCKs am» ii\>ti;i;i:sis function. 27 



that the strain produced by the shearing stress is of a very complicat- 

 ed nature. Though the greater part of the strain occurs almost 

 instantaneously, the total amount of it gradually increases with 

 time : i.e. say S=S +S t . When the couple is removed, the greater 

 part disappears instantaneously and the remaining part diminishes 

 gradually, and the limiting value for infinite time is generally sup- 

 posed not to be zero. Thus we have S=S' +S' t + S r . The suffixes 

 and t mean that the strain requires no time and a certain time 

 respectively to appear or disappear, while r means that it remains for 

 an infinite time. These facts have long been noticed by many 

 experimentalists, as cited in the introduction above. Several dis- 

 tinguished elasticians have endeavoured to establish a relation 

 between these different kinds of strains. 



Since there is no reason for assuming S =S' and also as S't 

 can not be equal to S t unless S r is zero, the general expression for the 

 strain must be of a form 



o = o 00 + O ot + O 0r + Of-o 4" &ft "1" o rr 



so that S =S . + S ot + S . r ; S' o =S o . + S t . o ; S t =S t . + S tt + S tr ; 



S' t = S ot + S t . t ; and S r =S . r +S t . r . 

 To establish a complete relation between stress, strain and time, we 

 must find a relation between the stress and each term of the strain 

 above mentioned. 



The simplest is the case where all the terms except the first are 

 negligibly small. Such a body is generally said to be perfectly 

 elastic. Within proper limits, which are called the limits of elasticity, 

 this is the case for most hard solids. As regards the relation between 

 stress and strain under this condition, Hooke's investigation was 

 most satisfactory and the result expressed in the law, well known by 

 his name — Hooke's law — is so closely associated with perfect elasticity 



