135 



LECTURE XIII. 



ON PASSIVE STRENGTH AND FRICTION. 



XriE passive strength of the materials employed in the mechanical arts de- 

 pends on the cohesive and repulsive forces of their particles, and on the 

 rigidity of their .structure. The consideration of the intimate nature of these 

 forces belongs to the discussion of the physical properties of matter; but the 

 estimation of their magnitude, and of their relative value in various circum- 

 stances, is of undeniable importance to practical mechanics, and 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 considerations. 



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

 Seven denominations; extension, compression, detrusion, flexure, torsion, 

 alteration, and fracture. When a Aveight 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 pri- 

 marily by a repulsive force, bvit secondarily also by its rigidity. The eflfect 

 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 principally the rigidity, 

 or lateral adhesion of the strata of the substance, but it could scarcely be 

 effectual without some degree of cohesive and repulsive 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 



