376 Proceedings of the Royal Society of Edinburgh. [Sess. 
elsewhere,* the author has found, from a large number of tensile tests of 
mild steel such as is used for structural purposes, that the usual value of the 
angle a is 50°, and that deviations from this inclination are generally quite 
small. The corresponding value of the angle /3 obtained in compressive 
tests should be 40°, but this has not been verified so satisfactorily. The 
value of the angle of friction is, then, <p = 2(« — 45°) = 10°, and the coefficient 
of friction is ^ = tan <^ = 0T76. The last figure agrees very closely with 
the coefficients obtained by Morin for clean, dry, plane, metallic surfaces. 
2. Relation between the Yield Points in Tension and Compression. 
A relation between the values of the stresses corresponding with the yield 
points in tension and compression can be found on the assumption that 
yielding takes place when the internal friction of the metal is just overcome. 
Let t be the stress at which permanent yielding occurs under a tensile 
load. 
Let c be the corresponding stress under a crushing load. 
If the limiting friction per unit of surface is the same in each case, 
* Proc. Inst. Mech. Eng ., Feb. 1905, p. 141. 
