156 LECTURE XIII. 



are detruded, or displaced, in opposite 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 tak- 

 ing a set. But the limit of all these effects is fracture, which is the conse- 

 quence of the application 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 di- 

 vided into two kinds, simple pressure, and impulse ; but it is only with re- 

 gard to fracture that it will be necessary to Lake 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 re- 

 pulsive forces, which resist these effects, depend almost as mucli 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 repidsive 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 particles 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 probably 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 and thickness are diminished. 



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

 ticles were prevented, the direct cohesion alone would be the measure of the 

 force required to produce extension, and the direct repulsion of the force 

 required to produce compression; in this respect indeed, the actual rigidity of 

 some substances may be considered as infinite, wherever the exten^jion or 

 compression is moderate, and no permanent alteration of form is produced; 

 and within these limits these substances may be called perfectly elastic. If 



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