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smooth oaken planks ; and it will scarcely seem to abate its velocity over a sheet of pure ice.
This short explanation of the nature and effects of collision and friction is sufficient to show, that smoothness and hardness are the chief qualities to be secured in constructing a road. But perfect smoothness cannot be obtained without first securing perfect hardness, and therefore the business of making a good road may be said to resolve itself into that of securing perfect hardness.
With the view of taking the right course for securing this object, the first thing a road trustee or engineer should do, is to form a correct notion of what hardness is; because the common habit of overlooking this circumstance has been the source of great error in forming opinions upon the qualities of different kinds of roads.
Gravel roads, for instance, to which an appearance of smoothness is given by incurring a vast expense in scraping them, and patching them with thin layers of very small gravel, are very commonly declared 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 it is practicable to produce it, 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 the crust of the road 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.
* i Bridges' Natural Philosophy, vol. i. p. 150.
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, 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 next must construct a crust of road materials in such a manner that, when consolidated, it shall
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 to lay upon the prepared bed of earth merely 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 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. 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, 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
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.