ON PASSIVE STREKGTH AND FRICTION. H3 



The last effect of force on solid materials is their fracture, which, as well as 

 the former changes, may be produced either by impulse, or by pressure alone. 

 The action which resists pressure is called strength, and that which resists 

 impulse may properly be termed resilience. The strength of every body is in 

 the joint ratio of its immediate cohesion and repulsion, or elasticity, and of 

 its toughness, or the degree in which it may be extended, compressed, or 

 otherwise deranged, without a separation of its parts. 'I he resilience is 

 jointly proportional to its strength and its toughness, and is measured by the 

 product of the mass and the square of the velocity of a body capable of break- 

 ing it, or of the mass and the height from which it must fall in order to ac- 

 quire that velocity ; while the strength is merely measured by the greatest 

 pressure that it can support in a state of rest. 



The simplest way in which a body can be broken, is by tearing it asunder. 

 The cohesive force continues to be increased as long as the tenacity of the 

 substance allows the particles to be separated from each other Avithout a per- 

 manent alteration of form; when this has been produced, the same force, if its 

 action is continued, is generally capable of causing a total solution of conr 

 tinuity; and sometimes a separation takes place without any previous altera- 

 tion of this kind that can be observed. 



It follows from the nature of resilience, that a body of a pound weight, 

 falling from the height of a yard, Avill produce the same effect in breaking 

 any substance, as a body of three pounds falling from the height of a foot ; 

 so that here, as well as in the estimation of mechanical power, it is the energy,- 

 and not the momentum, that is to be considered as the measure of the effect. 

 If we know the strength of any substance, and the degree in which it is ca- 

 pable of extension, we may easily determine its resilience from a consideration 

 of the laws of pendulums. For the same weight which would break it by 

 pressure, will acquire a sufficient impulse for breaking it, if it fall from a 

 height equal to half the space through which the substance may be extended, 

 supposing the direction of the stroke to be horizontal, so that its effect may 

 not be increased by the force of gravity. Thus if the pressure of a weight of 

 100 pounds broke a given substance, after extending it through the space of 

 an inch, the same weight would break it by striking it with the velocity that 

 •would be acquired by the fall of a heavy body from the height of half an 



