571 PRACTICE OF AGRICULTURE. Part III. 



pasture; and, indeed, other circumstances the same, a country abounding in hills and 

 valleys requires many more roads than one of a more even surface. The roads in such 

 a country are also moiv expensive, on account of the bridges, and extra work at their 

 abutments. On an estate composed of gentle hills chiefly intended for arable or con- 

 vertible husbandry, the best situation for the roads will generally be found about half way 

 between die bottoms and highest surfaces. By this means the labour of carting up the 

 produce from the fields below the road, and carting up the dung to the fields above it, is 

 evidently much less than if the road were either entirely on the highest ground or the 

 lowest. Bridges over die brooks or open ditches necessary for drainage in valleys, are 

 also rendered less frequent. 



:;."<;.!. Accurate sections of the rises and falls of the natural surface on which a road is 

 to be formed should always be taken before the line is finally determined on. As the 

 figure of on exact section of this sort, on any ordinary scale, would convey no data 

 sufficiently accurate for execution, it is usual to adopt one scale for the length, and 

 another for the rises and falls of the road, and to mark the latter with the dimensions as 

 taken on the survey. 



Sect. III. Form ami Materials of Roads. 



3564. 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 forming 

 them, and of the different kinds of road materials. 



Subsect. 1. Formation of Roads, and of their Wear or Injury. 



3565. 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 traffic ; of sufficient strength and 

 solidity for the given weight ; of sufficient smoothness to offer no impediment ; and of as 

 great durability as possible. 



3566. The width is obviously determinable by the nature and extent of the traffic : 

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

 which are in use in the country or district to pass each other ; and highways, 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 in Britain, and this includes the footpath. 



3567. 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 [joint 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 re- 

 sisting the impression of the feet of animals and the wheels of carriages in a great degree, 

 and impermeable 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 per- 

 haps in some cases by other means. 



a r >68. In cases of a wet or soft foundation, where from the nature of the soil and the pressure of the 

 springs lying on a higher level, as on the groat north road, near Highgate, draining has been found 

 ineffectual in drving the foundation of the road ; the same object has been attained by laying down, and 

 joining by cement, blocks composed of course gravel and Roman cement The water is thus prevented 

 from oozing up, and a foundation formed, at once firm, durable, and dry. This invention, with many 

 others in modern road-making, belongs to Mr. Telford. {Newton's Journal, vol ii, p. ~28.) 



3569. 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 they are 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 admitting 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 : hence 



