1907-8.] On the Cohesion of Steel, and Yield Points. 
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occurs along the surface where the difference between the intensity of 
tangential stress and the frictional resistance is greatest, that is, where 
p . sin ft . cos /3 —p . sin 2 /3 . tan 0 is a maximum ; that is, where /3 = 45° — 0/2. 
(b) Tension. (Fig. 2.) 
The intensity of tangential stress along a surface inclined at an angle 
a to the axis is p . sin a . cos a. 
The frictional resistance per unit of area along this surface is 
fx . p . sin 2 a —p . sin 2 a . tan 0. 
The frictional resistance decreases with increase of load, because the 
pressure between the surfaces of sliding is decreased. Hence, if increase 
of load is regarded as positive, the change in friction is a negative increase* 
Slipping occurs along the surface where p . sin a . cos a — ( — p . sin 2 a . tan 0) 
is a maximum ; that is, where a — 45° 4 - 0/2. It follows that a + /3 = 90°. 
Evidences of slipping in directions such as have just been indicated are 
afforded by the inspection of iron and steel bars which have been sub- 
mitted to loads exceeding the elastic limit of the material. As stated 
