Book II. 



WEAR OF ROADS. 



517 



3323. The wear or decay of roads takes place in consequence of the friction, leverage, 

 pressure, grinding and incision of animals and machines, and the various effects of 

 water and the weather. 



3324. Friction will in time wear down the most durable and smooth material. Its 

 effects are more rapid when aided by water, which insinuates itself among the particles 

 of the surfaces of earthy bodies, and being then compressed by the weight of feet or 

 wheels, ruptures or wears them. Even when not compressed by wheels or other weights, 

 the action of frost, by expanding water, produces the same effect. This any one 

 inay prove by soaking a soft brick in water and exposing it to a severe frost. 



3325. The leverage of the feet of animals has a 

 tendency to depress one part of the surface and 

 raise up another. The line which forms the 

 sole of every animal's foot may be considered as 

 a lever of the second kind, in which the fulcrum 

 is at the one extremity [fg. 435 a), the power 

 at the other (6), and the weight between them 

 {c). Hence the injury done to the road, even if 

 formed on the best construction, will be as the 

 pressure on the fulcrum : this amounts to the half of the weight of bipeds and their 

 loads, and a fourth of that of quadrupeds. But if the stones of the road are large, 

 that is, if they are more than two inches in breadth, the horse's foot acts as a compound 

 lever, and by depressing one end of the stones and raising the other, deranges the surface 

 of the stratum, and renders it a receptacle for water, mud, or dust. 



3326. The leverage of wheels is of a 

 nature to be less injurious to roads 

 than that of the feet of animals, be- 

 cause the fulcrum {fig. 436 a), is 

 continually changing its position. But 

 if the stones of the road are large, 

 then the wheel acts as a compound 

 lever, and raises up the one end (6), 

 and presses down the other (n), of 

 every stone it passes over, and in this 

 case becomes more injurious on a bad 

 road than the feet of loaded animals. 

 The reiterated operation of this effect 

 by wheels following in the same track, soon destroys badly constructed roads. 



3327. Such being the effect of leverage, and especially of compound leverage, in wearing 

 roads, it becomes of the first importance to ascertain that size and shape of stone on 

 which its effects will be least ; that is to say, how short a compound lever may be made use 

 of consistently with other advantages. This must in general be a matter of experience, 

 and chiefly depends on the hardness of the stone. The size must always be sufficiently 

 large, and the shape sufficiently angular to form, when embedded, a compact, hard, and 

 immoveable stratum, and the smaller the size the better, provided that object be obtained. 

 Two inches in diameter may be considered the medium size. 



3328. The mere j)ressure of objects on a smooth road does little mischief, and hence 

 the advantage of perfectly cylindrical wheels, and a road as nearly level as practicable. 

 But if the surface of the road is rough, the pressure both of cylindrical wheels, and the 

 feet of animals, may do mischief by forcing down a loose stone among others of dif- 

 ferent sizes, and thus loosening the latter and raising the largest to the surface. Where 

 a road, however, is composed of materials of small size, the pressure of cylindrical wheels, 

 when the surface is clean and dry, will probably always be of greater service by acting as 

 a roller, than of injury by the friction of the pressure. 



3329. Grinding is produced by the twisting motion of the feet of horses or other 

 animals when pulling hard or carrying a heavy weight, and by the twisting, dragging, 

 or sliding of wheels from whatever cause. The grinding of wheels, Fry observes, " may 

 in every case be defined to be the effect produced on any substance interposed between 

 two bodies, one of which has a sliding motion, yet so firmly confined or pressed between 

 them, that the moving body cannot slide over the interposed substance ; but, in conse- 

 quence of the pressure, the interposed substance, adhering firmly both to the fixed and 

 to the moving body, is necessarily lacerated or torn asunder, and reduced to atoms. 

 This is the process in corn-mills, in drug-mills, and in every other mill, properly so 

 called. I remember," he adds, " frequently when a boy, to have trodden with one heel 

 on a piece of soft brick, or of dry old mortar, which was firm enough to bear the weight 

 of my body, uninjured ; but, on giving my body a swing round with my other foot, I 

 have instantly reduced it to powder. The action in this case is very obvious : the 

 weight of my body confined the piece of brick firmly to the ground j my heel was also 



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