982 ON FORM AND MECHANICAL EFFICIENCY [ch. 



"angular distortion" in a figure, or (what comes to the same thing) 

 which tends to cause its particles to slide over one another, A 

 shearing stress is a somewhat complicated thing, and we must try- 

 to illustrate it (however imperfectly) in the simplest possible way. 

 If we build up a pillar, for instance, of flat horizontal slates, or of 

 a pack of cards, a vertical load placed upon it will produce com- 

 pression, but will have no tendency to cause one card to slide, or 

 shear, upon another; and in like manner, if we make up a cable 

 of parallel wires and, letting it hang vertically, load it evenly with 



Fig. 466. Trabecular structure of the os calcis. From MacAIister. 



a weight, again the tensile stress produced has no tendency to cause 

 one wire to slip or shear upon another. But the case would haye 

 been very different if we had built up our pillar of cards or slates 

 lying obliquely to the lines of pressure, for then at once there would 

 have been a tendency for the elements of the pile to slip and shde 

 asunder, and to produce what the geologists call "a fault" in the 

 structure. 



Somewhat more generally, if AB be a bar, or pillar, of cross-section a 

 under a direct load P, giving a direct and uniformly distributed stress per unit 

 area =p, then the whole pressure P=pa. Let CD be an oblique section, 

 inclined at an angle 6 to the cross-section ; the pressure on CD will evidently 

 be =pa cos 6. But at any point in CD, the pressure P may be resolved into 

 the shearing force Q acting along CD, and the direct force N perpendicular to 

 i t : where N = P cos 6 = pa cos 6, and ^ = P sin ^ = ^a sin 6. The shearing force 



