576 PRACTICE OF AGRICULTURE. Pam III. 



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



One inch in diameter may be considered tile medium size. 



3576. The nurepretmre of bodies 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 suses, and thus loosening the latter and raising the largest to the surface. Where 

 a road, however, is composed of materials of small size, and the surface is clean and dry, 

 the advantage derived from the pressure of cylindrical wheels acting as rollers will, it is 

 probable, always be greater than the injury sustained from their friction. 



3577, Grinding is produced by the twisting motion of the feet of horses or other 

 animals u lien 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. 1 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, 1 

 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 ; my heel was also 

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

 maining 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, which act only by pressure; by the use of conical wheels, which, by their constant 

 twist, act also by grinding ; and by very convex roads, by which means the wheels of all 

 carriages, except such as occupy the crown of the road, whether cylindrical or other- 

 wise, act in the same twisting, sliding, and grinding manner." (Obs. on Roads, Src. 



1819.) 



3578. By the incision of objects passing alongroads, we allude to the dividing operation 

 of 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 middle 

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



3579. Water is one of the most serious causes of the wear of roads. As we have 

 already observed (3572.), 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 injurious 

 earthy or vegetable matters. 



3580. Wind is mostly a favourable 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. M'Adams Theory and Practice of Road-maldng. 



3581. M'Adam agrees with other engineers, that a good road may be t-onsidered 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 carrying 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 is 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 origin- 

 ality of M'Adam's plan consists, unless we add also his assertion, " that all the roads in 



