ON PASSIVE STRENGTH AXD FUICTIOV. lAS 



to the other, and ascertain the degree in which it can have its dimensions 

 altered without fracture. 



It is easy to understand, from this statement, the different qualities of na- 

 tural bodies with respect to hardness, softness, toughness, and brittleness. 

 A column of chalk, capable of supporting only a pound, will perhaps be com- 

 pressed by it only a thousandth part of its length ; a column of elastic gum, 

 capable of suspending a pound, may be extended to more than twice its length, 

 the elastic gum will therefore resist the energy of an impulse incomparably 

 greater than the chalk. A diamond, so hard as to resist an enormous pres- 

 sure, may be broken, by a moderate blow, with a small hammer. A weight 

 of 1000 pounds, moving with a velocity of one foot in a second, and acting 

 on a small surface of a board, may possess sufficient energy to break or to 

 penetrate it; with a velocity of 100 feet in a second, a weight of .rV of ^ 

 pound will possess the same energy, and produce the same effect, if it act on 

 a similar surface; but if the wood be so constituted, as to be wholly incapable 

 of resisting a velocity of 100 feet in a second, it may be penetrated by a 

 weight of -r4-5- of a pound as well as by one tenth, and by a moderately soft 

 body as well as by a harder one. The whole board, however, if at liberty, 

 would receive a much greater momentum from the impulse of the large 

 weight, than from that of the small one, its action being continued for a 

 much longer time. And it is for this reason that a ball shot by a pistol will 

 perforate a sheet of paper standing upright on a table, without overturning it. 



The strength, or rather hardness, of a substance, exposed to the action of a 

 force that tends to compress it, must not be confounded with its resistance to 

 a force applied longitudinally and tending to produce flexure. A slender rod 

 of wood, when it yields to a longitudinal pressure, commonly bends before it 

 breaks, and gives way at last to the force by a transverse fracture ; but a 

 column of stone or brick, and even a thick pillar of wood, is crushed without 

 bending, and generally by a smaller force than that which would produce or 

 continue a flexure. In this case the parts slide away laterally, and in a rect- 

 angular pillar; if the texture of the substance is uniform, and not fibrous, 

 the surfaces of fracture will make nearly a right angle with each other, sup- 

 posing the resistance arising from the lateral adhesion, in the direction of any 

 surface or section, to be simply proportional to that section : but if this force, 



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