440 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
behaves like a viscous fluid, and flows laterally over the loaded faces of the 
test-piece. The friction between the lead and the stone, instead of tending 
to increase the apparent strength of the specimen, tends to decrease it. 
Let c" be the apparent yield point of the stone under these conditions, 
and let /3" be the inclination of the surfaces of sliding. Let p ( = tan <j>") 
be the coefficient of friction between the stone and the lead (fig. 6). 
The total resistance to shearing in this case is 
n • 9 n? f . ~T7~ ,f rf • /V t rvf tt ft • o otr 
\ic sim p + [iK. — i ifi c sm p cos (3 - [i c sim p , 
and the angle is such that 
n • o fr O ft • 9 n ff « ft n • ntt _ n't ft // • n 
c sm p cos p - 1 ic sue p + fi/u c sm p cos p + /n c sim p 
is a maximum. Hence 
tan 2/3 " = = cot (<f> - 
and 
j3" = 45° - J(^> - </>") = (3 + <f>" /2. 
As a rule, the stone does not rupture by shearing, but splits up in a 
different manner, discussed below. In the case of soft stones and concrete 
the pressure of fluidity of the lead may never be reached, and the inter- 
position of the metal may have no appreciable effect upon the strength of 
the specimen. On the other hand, for materials with crushing loads far 
