1(58

Conf ruction of Roads.—Effects of Wheels.

ciently opened and kept clear, on which accounts the roads fuffer confiderably,as from large quantities of water ftagnating upon them, deep floughs andruts are readily formed.

Roads have been fuppofed to be fubject to much injury and deftruetion fromfome particular forms of wheels ; thus broad ones, though conveying immenfeweights, have been recommended and encouraged, in preference to thofe of thenarrow kind, which carry comparatively but a very light load. But it is not thebreadth of the wheels which ought exclufively to be confidered, but the form orconftrudtion of them, and the weight of load that is placed upon and carried bythem ; as it is clear from the effedts which are produced, that the materials ofroads cannot be rolled down flat, or left in a perfectly folid ftate, by wheels pro-ceeding in Itraight directions, except where they have a cylindrical form of rim,and do not convey very weighty loads. The wheels of waggons, and other heavycarriages, though they have been required to have a certain breadth ofrim, and aflat bearing on the road, have not been conftrudted infuch a form as is heft fuitedto confolidate, roll down, and keep the furface of roads in repair. The foiesor rims of them, inftead of being cylindrical, have moftly been the portion of acone, the properties of which have lately, by various experiments, been (hewnto be thofe :—of their having a natural tendency, in rolling, to revolve in acircular direction round their conical centres ; of their requiring a conftantpower or force to keep them to a ftraight line or courfe ; in their being con-fined or compelled to move in fuch ftraight diredtion, a rubbing and frictionoccurring at the rim ; their augmenting fridtion on the axis; their caufing arubbing agamft the fidcs of deep ruts or tracks ; their throwing up dirt fromthe hind part of the wheel ; their pulverizing and greatly reducing the bedforts of materials in dry feafons, thereby caufing much fludge in wet, and muchduft in dry weather; their deranging and breaking the texture of the furface ofthe roads w'hen in a foft or compreftible ftate, and leaving them in a brokencondition, ready to imbibe moifture, which caufes all the bad effedts of wetfeafons and intenfe frofts ; their promoting the deftrudiion of paved roads, byforcing open the joints and letting in the w'ater under the ftones, which byultimately floating and difeharging the gravel, renders the ftones loofe andpermits the pavement to fink into holes ; their augmenting the labour of thecattle or team, and accelerating the wear of the tyres of the wheels by theirconftant tendency to drag and grind on the roads *.

The cylindrical form of rims is (hewn to be free from thefe inconveniences,having a conftant tendency to proceed in a ftraight diredtion, without friction

* Cumming in Communications to the Board of Agriculture, vol. II.