and, at the same time, press much of the earth upwards between the stones; this will take place to a great degree in wet weather, when the bed of earth will be converted into soft mud by water passing from the surface of the road, through the broken stones, into the bed of the road. In this way a considerable quantity of earth will be mixed with the stone materials laid on for forming the crust of the road, and this mixture will make it extremely imperfect as to hardness. It might be possible, in some measure, to cure this defect by laying on a succession of coatings of broken stones; but several of these will be necessary, and, after all, in long continued wet weather, the mud will continue to be pressed upwards from the bottom to the surface of the stones. even a coating of from sixteen to twenty inches of stones be laid on, it will produce only a palliative of the evil. So that this plan of making a road will be not only very imperfect, but at the same time very expensive. If Mr. Telford's plan, which has completely succeeded on the Holyhead Road, the Glasgow and Carlisle Road, and several other roads in Scotland, of making a regular bottoming of rough, closeset pavement, is a plan that secures the greatest degree of hardness that can be given to a road ; it is also attended with much less expense than when a thick coating of broken stones is used; for six inches of broken stones is sufficient when laid on a pavement, and the pavement may be made with any kind of common stone. By laying the stones in making the bottoming with their broadest face downwards, and filling up the interstices closely with stone chips well driven in, the earthy bed of the road cannot be pressed up so as to be mixed with the coating of broken stones. This coating, therefore, when consolidated, will form a solid uniform mass of stone, and be infinitely harder than one of broken stones, when mixed with the earth of the substratum of the road. It is by proceeding in the way here recommended that the friction of wheels on a road will be reduced as much as possible.* To comprehend thoroughly the great importance of making a regular and strong foundation for a road, it should be borne in mind, that roads are structures that have to sustain great weights, and violent percussion; the same rules therefore ought to be followed in regard to them as are followed in regard to other structures. *The mathematical illustration of the effect of friction on carriages is given in note C. In building edifices which are to support great weights, whether a church, a house, or a bridge, the primary and indispensable consideration of the architect is to obtain a permanently firm and stable foundation; well knowing that unless this be first substantially made, no future dependence can be placed on the stability of the intended superstructure: but this most requisite precaution has but recently been attended to in the formation of roads, and only on those roads in Scotland, and between London and Holyhead, which have been under the direction of Mr. Telford. If the foundation of a road be not sufficient and equal to the pressure it has to sustain, the whole fabric, though in other respects ever so well constructed, must fail in permanent stability, and the hardness of it will be imperfect from its elasticity. Having now stated all that the rules of science relating to moving bodies suggest, in order to defend the principles of road making, which have been laid down as the proper principles to be adopted, we shall now proceed further to illustrate and support these principles, by referring to experiments of the force of traction on different kinds of roads. These experiments have been made with the machine invented by Mr. Macneill, which has been already mentioned, and may be relied upon for their accuracy, in consequence of their having been carefully examined by several very eminent civil engineers. These experiments uniformly show, that the force of traction is, in every case, nearly in an exact proportion to the strength and hardness of a road. The following are the results: on a wellmade pavement, the power required to draw a waggon is 33 lbs. ; on a road made with six inches of broken stone of great hardness, laid on a foundation of large stones, set in the form of a pavement, the power required is 46 lbs. ; on a road made with a thick coating of broken stone, laid on earth, the power required is 65 lbs.; and on a road made with a thick coating of gravel, laid on earth, the power required is 147 lbs.* Thus it appears that the results of actual experiments fully correspond with those deduced from the laws of science. It has been considered necessary to enter into these details in showing that no road can be correctly called a good road unless it is so constructed as to be a very strong and a very hard one, be * See Appendix, No. I. cause all the main roads of the kingdom are still very defective in respect to strength and hardness. This is a fact which cannot be disputed; first, because there is always mixed up with the body of materials, which forms the crust of every road, a great quantity of earth; secondly, because this crust is every where too thin; and, thirdly, because it very frequently lies upon an elastic substratum. Although there may be exceptions, may be taken as an accurate description of the general state of the roads. this Notwithstanding all the roads are now much better than at any former period, and may deserve to be called good roads, in comparison with the roads of ten or fifteen years ago; years ago; when it is considered how much better they would be if they were reconstructed with a proper foundation coated with broken stones of great hardness, they should still be set down as being imperfect roads. Let any road trustee or surveyor who doubts this, reconstruct a mile of a road, now considered an excellent one, with a bottoming of pavement, coated with hard stones, and no stage coachman who shall drive over it will hesitate to bear testimony to the increased ease with which his horses do their work upon it. |