146 LECTURE XIII. 



like that of friction, is increased by a pressure which tends to bring the parts 

 into closer contact, the angle left after fracture must be more acute. (Plate 

 X. Fig. 124.) 



The power of the force of lateral adhesion, in resisting fracture, is consider- 

 ed by Mr. Coulomb as nearly equal to that of the direct cohesion of the same 

 substance, or a little greater; while Professor Robison makes it twice as 

 great. If, however, this force be supposed to be simply equal to the direct 

 cohesion, it may be inferred that the strength of a square bar in resisting 

 compression is twice as great as its cohesive strength, allowing that the frac- 

 ture takes place in the surface of least resistance. It is, however, seldom 

 that the strength, with which a body resists compression, is in so great a pro- 

 portion as this to its cohesive strength; and where the substance is in any 

 degree composed of fibres, they must naturally produce great irregularities by 

 their flexure. The strength in resisting compression, must, according to 

 this statement, be simply proportional to the magnitude of the section of the 

 substance, although some experiments on freestone appear to indicate that 

 when the section is increased, the strength is increased in a greater proportion; 

 jind there is no reason to suppose that it can be influenced either way by the 

 length. A cylindrical or prismatic form is therefore the best that can be 

 given to materials of a given bulk, in order to enable them to resist a force 

 which tends to crush them, except tliat the additional pressure of their own 

 weight on the lower parts, requires that those parts should be a little stronger 

 than the upper parts. It appears also that something is gained by making 

 the outline a little convex externally: for it may be demonstrated, that for a 

 column or upright beam, to be cut out of a slab of equable thickness, supposing 

 the strength to be independent of pressure, the strongest form is a circle. 

 (Plate IX. Fig. 126, 127.) 



When a body is broken by a transverse force applied very near to a fixed 

 point, its lateral adhesion is overpowered by the effect which we have called 

 detrusion, and its strength in this case is therefore generally somewhat greater 

 than its direct cohesive strength. But when the part to which the force is 

 Immediately applied is at a distance from the fixed point greater than about 

 one sixth of the depth, the fracture is no longer the immediate consequence 

 of detrusion, but of flexure. 



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