SIB PRACTICE OF AGRICULTURE. Part III 



Sect. III. 0/" the Form and Materials of Roads. 



3317. On the structure and composition of roads, men of science and practical road- 

 makers are much more divided than on their laying out. The subject is of itself of 

 greater importance in old countries, because it more frequently occurs that a road is to 

 be enlarged or renewed, than that a new line is to be devised. We shall first lay down 

 the fundamental principles of the formation, and wear of roads, and next treat of form- 

 ing them, and of the different kinds of road materials. 



SuBSECT. 1. Of the Formation of Roads, and of their Wear or Injury. 



^ 3318. A road may be defined a path of transit on the earth's surface, for men, 

 animals, and machines ; of sufficient width for the given traffick ; of sufficient 

 strength and solidity for the given weight; of sufficient smoothness to permit no 

 impediment ; and of as great durability as possil)le. 



3319. The width is obviously determinable by the nature and extent of the traffick : 

 every road should be made sufficiently broad to admit two of the largest sized carnages 

 which are in use in the country or district, to pass each other ; and higliways and roads 

 near towns should be made wider in proportion to their use. The maximum and 

 minimum can only be determined by experience : sixty feet is the common and legal 

 width of a turnpike-road iw Britain, and this includes tlie footpath. 



3320. The strength of a road depends on the nature of the material of which it is 

 formed, and of the basis on which it is placed. A plate of iron or stone of the road's 

 width placed on a compact dry soil would comprise every thing in point of strength ; but 

 as it is impracticable to employ plates of iron or stone of such a size to any extent, 

 recourse is had to a stratum of small stones or gravel. The great art, therefore, is so to 

 prepare this stratum, and place it on the basis of the road, as that the effect may come 

 as near as possible to a solid plate of material. To accomplish this, the stones or 

 gravel should be broken into small angular fragments, and after being laid down of such 

 a thickness as experience has determined to be of sufficient strength and durability, the 

 whole should be so powerfully compressed by a roller as to render it one compact body, 

 capable of resisting the impression of the feet of animals, and the wheels of carriages 

 in a great degree, and imperveable by surface water. But the base of the road may 

 not always be firm and compact ; in this case it is to be rendered so by drainage, 

 artificial pressure, and perhaps in some cases by other means. 



3321. The durability of a road as far as it depends on the original formation, will be 

 in proportion to the solidity of its basis ; the hardness of the material of which the 

 surface stratum is formed ; its thickness ; and the size and form of the stones 

 which compose it. The form and size of the stones which compose the surface-stratum 

 have a powerful influence on a road's durability. If their form is roundish, it is evident 

 they will not bind into a compact stratum ; if it is large, whether the form be round or 

 angular, the stratum cannot be solid ; and if they are of mixed sizes and shapes, though 

 a very strong and solid stratum may be formed at first, yet the wheels of carriages and 

 the feet of animals operating with unequal effect on the small and large stones, would 

 soon derange the solidity of the stratum to a certain depth, and consequently, by admit- 

 ting rain and frost to penetrate into it, accelerate its decay. A constant state of 

 moisture, even without any derangement of surface, contributes to the wearing of roads 

 by friction, and hence one requisite to durability is a free exposure to the sun and air 

 by keeping low the side fences ; and anothei- is keeping a road clear of mud or 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 & road with a regular bearing on the soles of its feet ; and, 

 secondly, because no wheeled carriage can have a regular bearing, excepting on the 

 middle or crown of the road. The consequence of both these states is the breaking 

 the surface of the plate into holes with the edges of horses' feet, or ruts by the plough- 

 like effect of wheels on the lower side of the road, or the reiterated operation of those 

 which pass along the centre. 



,3322, 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 con- 

 tinued 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 wheels or water penetrates from 

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

 loose stones. 



