518 PRACTICE OF AGRICULTURE. Part III. 



pressed by the same weight firmly upon the brick ; one part of the brick therefore remain- 

 ing confined to the ground, and the other part being carried round by my heel, the 

 brick of course was torn asunder and reduced to powder. This I conceive is a simple 

 elucidation of the difference between pressing and grinding, and this is the difference 

 of the effects on the materials of our roads, produced by the use of upright cylindrical 

 wheels, whicli act only hy pressure, by the use of conical wheels, which by their constant 

 twist, act also by grindingy and by very convex roads, by which means the wheels of 

 all carriages, excepting such as occupy the crown of the road, whether cylindrical or 

 otherwise, act in the same twisting, sliding, and grinding manner." (06s. on Roads, ^c. 

 1819.) 



3330. By the incision of objects jmssing along roads, we allude to the dividing operation 

 t)f wheels, which, independently of their effect as moving levers, act also as moving 

 wedges, or perhaps, more properly, as endless saws in forming ruts or deepening such 

 as are already made. Flat roads, so as to produce less temptation to follow in the mid- 

 dle track, watchful repair, and broad wheels, are the mitigators of this description of 

 wear. 



3331. Water is one of the most serious causes of the wear of roads. As we have 

 already observed (3324.), it acts, aided by pressure, like gunpowder in rending the sur- 

 face of bodies. Frozen it acts exactly in the same manner; and when it has penetrated 

 deeply into a stratum of materials, a thaw produces their entire derangement. Mud is 

 formed in consequence of the presence of water and dust, or earth, and acts as a sponge 

 to retain it, and perpetuate its bad effects. A well-composed and thoroughly com- 

 pressed substratum will not imbibe water unless it rests in ruts or other hollows. To 

 form such a stratum, therefore, and obliterate all hollows as soon as they appear, and 

 to remove mud and dust, are the palliatives of this mode of wear. On such a road 

 heavy showers may do good by washing away the earthy particles, dung, and other in- 

 jurious earthy or vegetable matters. 



3332. Wind is mostly a favorable agent to roads by drying them and blowing off the 

 lighter dust ; but in some cases, in very exposed situations, it has been known to blow 

 the dust into heaps, and sometimes to carry off" larger particles than could be spared. 

 The last evil is fortunately rare ; the other only requires the removal of the accumulated 

 heaps of dust. 



SuBSECT. 2. Of M^Adani's Theory and Practice of Road-making. 



3333. M^Adam agrees with other engineers, that a good road may be con- 

 sidered as an artificial flooring, forming a strong, solid, smooth, surfaced stratum, 

 sufficiently flat to admit of carriages standing upright on any part of it, capable of car- 

 rying a great weight, and presenting no impediment to the animals or machines which pass 

 along it. In forming this flooring, M'Adam has gone one step beyond his predecessors 

 in breaking the stone to a smaller size than was before practised, and in forming the 

 entire stratum of this small-sized stone. By the former practice a basement of large 

 stones are first laid, then stones a degree smaller, and, lastly, the least size on the surface. 

 It is in this point of making use of one small size of stones throughout the stratum, 

 that the originality of M' Adam's plan consists, unless we add also his assertion, " that 

 all the roads in the kingdom may be made smooth and solid in an equal degree, and to 

 continue so at all seasons of the year." It is doubted by some, whether this would be 

 the case in the northern districts at the breaking up of frosts, and especially in the case 

 of roads not much in use, and consequently consisting of a stratum less consolidated, 

 and more penetrable by water. M'Adam, probably, has much frequented pul)lic 

 roads in view. ** The durability of these," he says, " will of course depend on the 

 strength of the materials of which they may be composed, but they will all be good 

 while they last, and the only question that can arise respecting the kind of materials, is 

 one of duration and expense, but never of the immediate condition of the roads." (j?e- 

 marks on Roads, ^c. p. 11.) The following observation of Marshal is worthy of re- 

 mark, as tending to confirm to a certain extent the doctrine of M'Adam. " It may 

 seem needless to repeat, that the surface of a road which is formed of well-broken 

 stones, binding gravel, or other firmly cohesive materials, and which is much used, pre- 

 sently becomes repellent of the water which falls upon it : no matter as to the basis on 

 which they are deposited ; provided it is sound and firm enough to support them." 



3334. M'Adavis theory of road-making may be comprised in the following quotation 

 from his Report to the Board of Agriculture (vol. vi. p. 46.). *' Roads can never be 

 rendered perfectly secure until the following principles be fully understood, admitted, 

 and acted upon : namely, that it is the native soil which really supports the weight of 

 traffick ; that while it is preserved in a dry state it will carry any weight without sinking, 

 and that it does, in fact, carry the road and the carriages also ; that this native soil must 

 previously be made quite dry, and a covering impenetrable to rain, must then be placed 

 over it to preserve it in that dry state ; that the thickness of a road should only be regu- 



