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to be perfect, and unequalled by any other kind of road. But if the best gravel road be compared with one properly constructed with stone materials, the hardness of the former will be found to be greatly inferior to that of the latter, and the error of the advocates of smooth-looking gravel roads will be immediately made manifest.

By referring to works of science, it will be seen that hardness is defined to be that property of a body by which it resists the impression of other bodies which impinge upon it; and the degree of hardness is measured by the quantity of this resistance. If the resistance be so complete as to render it totally incapable of any impression, then a body is said to be perfectly hard.*

Now this hardness is the hardness which a road ought to have as far as is practicable, and it is the chief business of a scientific road maker to do every thing necessary to produce it. For this purpose, when making a new road, he should first select or establish a substratum of soil or earth that is not spongy or elastic, for the bed of the road; and then he should so dispose the materials of which its crust is to consist, as to form a body sufficiently strong to oppose the greatest possible quantity of resistance to the weight of heavy carriages passing over it.

That an elastic subsoil is unfit for a road is evident from the nature of the resistance occasioned by friction, as above described by Professor Leslie,

* Bridges' Natural Philosophy, vol. i. p. 150.

and from the terms of the definition of hardness; for however strong the crust of materials may be which is formed over such a subsoil, it will not be capable of opposing a perfect resistance to a heavy moving body. The moving body will sink more or less in proportion as the subsoil is elastic, and the hardness of the road will be imperfect in proportion as this sinking takes place; so that nothing can be more necessary, as a preliminary step in making a new road, than to take every possible precaution to avoid elastic subsoils, or to destroy the elasticity as much as possible, when no other can be found.

After the engineer has prepared a proper substratum of earth for the bed of a road, he must next construct a crust of materials in such a manner as, when consolidated, to possess such a degree of hardness as will not admit the wheels of carriages to sink or cut into it. For this purpose it will not be sufficient merely to lay upon the prepared bed of earth a coating of broken stones; for the carriages passing over them will force those next the earth into it, 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 it. In this way a considerable quantity of earth will be mixed with the stone materials forming the crust of the road, and this mixture will make it extremely imperfect as to hardness, for it cannot, in fact, be perfectly hard unless it consists wholly of stones. 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. If 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.

Mr. Telford's plan of making a regular bottoming of rough, close-set pavement, which has completely succeeded on the Holyhead Road, the Glasgow and Carlisle Road, and several other roads in Scotland, is one that secures the greatest degree of hardness; 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.

If the stones in making the bottoming are laid with their broadest face downwards, and the interstices are filled 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 the surface of a road will be reduced as much as possible.

To comprehend thoroughly the great importance of making a regular and strong foundation, 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 with them as are followed with regard to other

structures.

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. He well knows 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 its hardness will be imperfect on account of its elasticity.

Having now stated all that the rules of science relating to moving bodies suggest, in order to de

The mathematical illustration of the effect of friction on carriages is given in note C.

fend the principles of road-making, which have been laid down as those proper to be adopted, we shall proceed further to illustrate and support these principles, by reference 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 which may be relied upon for their accuracy, in consequence of their having been carefully examined by several eminent civil engineers.

These experiments uniformly show, that the force of traction is, in every case, in an exact proportion to the strength and hardness of a road. The following are the results: on a well-made 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.*

* The following is an extract from the evidence of Mr. Macneill before the Committee on Turnpike Roads (1836):

"Are you still satisfied of the principle of your machine being a correct one, as to ascertaining the draughts of carriages?— I am quite certain of its being perfectly correct, for we have tried it in some cases, and compared it with a weight hanging over a pully, and the results were the same. It is only in cer

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