ON PASSIVE STRENGTH AND FRICTION. 105 



requires to be examined as a continuation of the subject of statics. The 

 retarding force of friction is very nearly allied to some kinds of passive 

 strength, and may be in great measure explained from similar conside- 

 rations. 



The principal effects of any force acting on a solid body may be reduced 

 to seven denominations ; extension, compression, detrusion, flexure, tor- 

 sion, alteration, and fracture. When a weight is suspended below a fixed 

 point, the suspending substance is extended or stretched, and retains its 

 form by its cohesion assisted by its rigidity : when the weight is supported 

 by a block or pillar placed below it, the block is compressed, and resists 

 primarily by a repulsive force, but secondarily also by its rigidity. The 

 effect here called detrusion is produced when a transverse force is applied 

 close to a fixed point, in the same manner as the blades of a pair of 

 scissors act on the pin, and the force which resists this operation is prin- 

 cipally the rigidity or lateral adhesion of the strata of the substance, but 

 it could scarcely be effectual without some degree of cohesive and repul- 

 sive force. When three or more forces are applied to different parts of 

 any substance they produce flexure, that is, they bend it, some of its 

 parts being extended and others compressed. In torsion or twisting, the 

 central particles remain in their natural state, while those which are 

 in opposite parts of the circumference are detruded or displaced, in op- 

 posite directions. The operation of forces applied in any of these ways 

 may produce a permanent alteration or change of figure in substances 

 sufficiently soft, and perhaps, in a certain degree, in all substances : this 

 change is sometimes called by workmen settling or taking a set. But the 

 limit of all these effects is fracture, which is the consequence of the appli- 

 cation of any force capable of overcoming the strength of the substance, 

 and to which the generality of writers on mechanics have hitherto confined 

 their attention. 



The forces by which the form of any substance is changed may also be 

 divided into two kinds, simple pressure and impulse ; but it is only with 

 regard to fracture that it will be necessary to take the force of impulse 

 into consideration. 



Extension and compression follow so nearly the same laws, that they 

 may be best understood by comparison with each other. The cohesive and 

 repulsive forces which resist these effects, depend almost as much on the 

 solidity or rigidity of the substances, as on the attractions and repulsions 

 which are their immediate causes : for a substance perfectly liquid, 

 although its particles are in full possession of their attractive and repulsive 

 powers, may be extended or compressed by the smallest force that can be 

 applied to it. It is not indeed certain that the actual distances of the par- 

 ticles of all bodies are increased when they are extended, or diminished 

 when they are compressed : for these changes are generally accompanied 

 by contrary changes in other parts of the same substance, although pro- 

 bably in a smaller degree. We may easily observe that if we compress a 

 piece of elastic gum in any direction, it extends itself in other directions ; 

 and if we extend it in length, its breadth arid thickness are diminished. 



If the rigidity of a body were infinite, and all lateral motions of its 



