Book II. 



WEAR OF ROADS. 



575 



one requisite to durability is a free exposure to the sun and air, by keeping low the side 

 fences ; and another is keeping a road clear of mud and dust the first of which acts as a 

 spunge in retaining water, and the second increases the draught of animals, and of course 

 their action on the road. Both the strength and the durability of a road will be greater 

 when the plate or surface-stratum of metals is flat or nearly so, than when it is rounded on 

 the upper surface : first, because no animal can stand upright on such a road with a 

 regular bearing on the soles of its feet ; and, secondly, because no wheeled carriage can 

 have a regular bearing, except on the middle or crown of the road. The consequence 

 of both these states is the breaking of the surface of the plate into holes from the edges 

 of horses' feet, or ruts from the plough-like effect of wheels on the lower side of the road, 

 or the reiterated operation of those which pass along the centre. 



3570. The smoothness of a road depends on the size of the stones, and on their com- 

 pression either by original rolling or the continued pressure of wheels. The continued 

 smoothness of a road during its wear depends on small stones being used in every 

 part of the stratum ; for if the lower part of it, as is generally the case in the old style 

 of forming roads, consists of larger stones, as soon as it is penetrated by wheels or water 

 from above, these stones will work up and produce a road full of holes and covered with 

 loose stones. 



3571. 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. 



3572. 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 may 

 prove, by soaking a soft brick in water and exposing it to a severe frost. A road in a 

 state of perfect dryness is, under the action of wheels, as liable to be injured in its solidity, 

 as when too wet; because it loses its elastic tenacity under the pressure, and becomes 

 broken into a loose superstratum. This is the greatest advantage of watering roads, as 

 proved by the experience of trustees, and shown in their annual accounts of expenses ; 

 besides the comfort to travellers, of laying the dust, for which alone watering was first 

 thought necessary. 



3573. The leverage of the feet of animals has a tendency to depress one part of the sur- 

 face 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 [fig. 539. a), the power at 

 the other (li), 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 



y^ on the fulcrum : this amounts to from the half to 

 the whole of the weight of bipeds and their loads, 

 and from a fourth to a half 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 com- 

 pound 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. 



3574. The leverage of ivheels is of a nature to be less injurious to roads than that 



of the feet of animals, because the 



540 fulcrum (fg. 540. a), is continually 



changing its position : but if the stones 

 of the road are large, then the wheel 

 acts as a compound lever, raising up 

 the one end (6), and depressing 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 

 j operation of this effect, by wheels fol- 

 lowing in the same track, soon destroys 

 badly constructed roads. 



3575. Such being the effect of leverage, and especially of compound leverage, in wearing 

 roadSf 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 suflSciently 

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



