STRESSES. 



11 



dimensions ; but, if a lump of moist clay be squeezed so as 

 to change its shape, on the removal of the squeezing force 

 there will be no springiness or tendency to return to its 

 former shape. 



The semi-plasticity of metals is made great use of in the 

 trades, as has already been pointed out. Wire is drawn, 

 hot iron and steel are forged, copper is beaten, and all 

 manner of small parts are formed from cold metal by 

 stamping. This property in the metal is called its 

 capability for taking "permanent set." 



4. External and Internal Stresses. If a body is acted 

 upon by any stress or system of stresses whatsoever, the 

 stresses so acting upon the body, and which are due to 

 causes outside the body, are termed external stresses; they 

 produce by their action certain effects within the internal 

 structure of the substance, tending to interfere with its 

 molecular arrangement ; these are the " internal stresses " 

 brought about by the " external stresses." 



In Fig. 1. A B C D is a cube. If a force P be applied 

 normal to one of its faces A E F B, and an equal and 

 opposite force also axially and normal to the opposite 

 face C G D, both forces acting away from the cube, a stress 

 in tension or tensile stress is said to be applied, and its 

 tendency is to cause rupture of the cube across some section, 

 such as a b c d at right angles to the direction of the 

 force, the force exerted on this section balancing the 



pull of the external force P. The area of the section 



