CHAPTER V. 



MECHANICAL PRINCIPLES. 



IN the previous chapter it has been shown that the appli- 

 cation of force as tension, compression, or shear, produces 

 strain among the particles of which the body consists, and that 

 this external pressure is resisted by the cohesive force of its 

 fibres; also that the internal resistance of the particles depends 

 upon their number and their arrangement in the cross-section. 

 When weight or pressure is applied to such body as a beam 

 or girder, two opposing forces are set in motion; one tending 

 to cause rupture or the breaking of the beam through its cross- 

 section, and the other exerting an opposing force of the fibre 

 resistance depending in effect upon arrangement and tenacity. 

 The tendency of the load applied to the beam is to produce 

 " flexure" or bending, straining the fibres on the under side 

 of the beam or producing tension among them, and compress- 

 ing correspondingly the upper or outside fibres, both directly 

 as their distance from the outer sides toward the centre of the 

 beam. The strain which taxes to the maximum those most 

 remote fibres from the central line, both by tension and com- 

 pression, is gradually neutralized as the strain of tension and 

 compression approach each other, and at the line of the cross- 

 section where these two opposing forces meet, the fibres are at 

 rest as regards each other, or are said to be in equilibrium, and 

 at that line the fibres are neither under tension nor compression. 



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