152 tECTURE XIIX. 



and firmer on the south east side of the tree. Although iron is much stronger 

 than wood, yet it is more liable to accidental imperfections; and when it 

 fails, itgives no warning of its approaching fracture. The equable quality of 

 steel may be ascertained by corrosion in an acid; but there is no easy mode 

 of detecting internal flaws in a bar of iron, and we can only rely on the ho- 

 nesty of the workman for its soundness. Wood, when it is crippled, com- 

 plains, or emits a sound, and after this, although it is much weakened, it 

 may still retain strength enough to be of service. Stone sometimes throws 

 off small splinters when it is beginning to give way: it is said to be capable 

 of supporting by much the greatest weight when it is placed in that position, 

 with respect to the horizon, in which it has been found in the quarry. 



It is obvious that when the bulk of the substance employed becomes 

 very considerable, its weight may bear so great a proportion to its strength 

 as to add materially to the load to be supported. In most cases the weight 

 increases more rapidly than the strength, and causes a practical limitation 

 of the magnitude of our machines and edifices. We see also a similar limit 

 in nature: a tree never grows to the height of 100 yards; an animal is never 

 strong enough to overset a mountain. It has been observed that whales are 

 often larger than any land animals, because their weight is more supported by 

 the pressure of the medium in which they swim. 



The force of friction, which resists the sliding of different bodies on each 

 other, seems to be intimately connected with that lateral adhesion, or rigidity, 

 which is opposed to the internal displacement of the parts of a single body, by 

 the effect which we have denominated detrusion : and when the friction is con- 

 sidered as resisting pressure rather than motion, it approaches still more nearly 

 to the same force. It is probably derived in great measure from the strength of 

 the protuberant particles, which must be broken, bent, or compressed by the 

 motion of the bodies on each other: but it is not always that the existence 

 of such particles can be asserted, much less can they be made perceptible to 

 the senses, and we can only examine the effects which they may be supposed 

 to produce, by immediate experiments on the forces required to counteract 

 them. Such experiments have been made on a very extensive scale by 

 Musschenbroek and Coulomb, and many of their results have been confirmed 

 by Mr. Vince, in a simple and elegant manner. 



