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stopped by the action of some external force. This proposition is admitted and adopted by all natural philosophers as being perfectly true, and therefore, in order to apply it to roads, it is necessary to enquire what species of external force act in a manner to diminish and destroy the momentum of carriages passing over them. With respect to these external forces, the general doctrine is, that they consist of, ist, collision ; 2d, friction ; 3d, gravity; and 4th, air.*
1st. The effect of collision is very great in diminishing the momentum of carriages ; it is occasioned by, and is in proportion to, the hard protuberances and other inequalities on the surface of a road. These occasion, by the resistance which they make to the wheels, jolts and shocks, which waste the power of draught, and considerably check the forward motion of a carriage.
The mathematical illustration of the effect of collision in producing this resistance is given in note B.
2d. Friction has very great influence in checking the motion of a carriage; for, when the wheels come into contact with a soft or elastic surface, the friction which takes place operates powerfully in obstructing the tendency of the carriage to proceed; the motion forwards is immediately retarded, and would soon cease if not renewed by the efforts of the horses. The “resistance,” Professor Leslie says,
“ which friction occasions, partakes of the nature of the resistance of fluids; it consists of the consumption of the moving force, or of the horse's labour, occasioned by the soft surface of the road and the continually depressing of the spongy and elastic sub-strata of the road.”
* See Wood's Mechanics, p. 20.
An ivory ball, set in motion with a certain velocity over a Turkey carpet, will suffer visible relaxation of its course; but, with the same impelling force, it will advance farther if rolled over a superfine cloth; still farther over smooth oaken planks ; and it will scarcely seem to abate its velocity over a sheet of
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 in a road. But perfect smoothness cannot be obtained without 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 scraping them, at a vast expense, 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.
* Elements of Natural Philosophy.
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 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
In this way a
If even a
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. 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