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is out of shape and where it gets weak, and they instantly repair it; by this means there is not the same jolting, the same degree of resistance, to carriages passing over it; and nothing but a perfect and uniform line of curves and levels will enable the eye of a workman to see where the deficiency takes place.
"Generally speaking, the roads are not uniform as to breadth, convexity, or width ? — No; there does not appear to be any system in this country on this very important point; generally speaking, no road that I am acquainted with has uniform width and height of footpath, and curvature of surface, even for half a mile in length.
"Would it contribute to the good order and keeping up of a road to pay attention to these points ? — It would be a great saving.
"A road is easier kept clean and dry ? — Yes, and more easily seen where it becomes weak.
"Are the workmen more attentive and careful when it is all put in a proper shape ? —They would become of a different character; a workman, as soon as he got a uniform and neat road, would have a pride in this work, and would keep it in better order and free from ruts, weeds, &c."
Before proceeding to examine what are the right principles for constructing roads, it will be useful to show correctly what is the proper object to be secured in making a road. It is owing very much to omitting to take this circumstance into consideration, that vague and erroneous opinions so generally prevail respecting what a road ought to be in order that it should be a perfectly good one. Almost all road-makers who have acquired reputation with the country gentlemen who are the trustees of roads, have acquired it under the idea that smoothness was the sole object to be secured, without reference to the indispensable necessity of making a road extremely hard and solid as well as smooth. Dr. Lardner says,—" Until a comparatively late period a very prevalent, indeed almost universal error obtained with respect to roads. All that people considered was what they conceived to be an easy motion to the passengers; that which was easiest to the passengers was concluded to be also the easiest to the horses; or perhaps I should be more correct in saying the horses were not considered at all. People never thought of taking into consideration the mechanical force which was necessary to draw a load along a road. If there were two roads with surfaces equally smooth, (acclivities of an equal steepness, and along which the passengers felt themselves equally comfortable,) those roads were at once assumed to be, to all intents and purposes, equally good; a greater mistake could scarcely be found than that. Suppose a road surface were made of Indian rubber, the surface being as smooth as it can be imagined to be, no road could be worse for traction; the wheels would sink into the surface, and the tractive force would be continually pulling up a hill; it would have the effect of a continual ascent. The surface of the road should be as hard and as unyielding as art can make it; the wheel should not sink; no temporary depression should take place, even though that depression be restored by elasticity after the wheel is removed. By whatever means, this end must be attained, it is quite essential, although there may be a difference in the means of attaining it; but attained it most certainly ought to be."
"The main object of a road connecting two places is to enable loads to be transported from the one place to the other in the least possible time, and with least possible expenditure to tractive power. This tractive power depends upon several qualities in the road; first, upon its levelness; secondly, upon the smoothness of its surface; and thirdly, upon a quality which may be called hardness, the absence, in fact, of elasticity."
One of the most important and most obviously correct principles of road-making is that which requires a road to be made of a substance in due proportion to the weight and number of the carriages that are to travel over it.
But although this is, in appearance, a self-evident proposition, no rule in practice is so universally violated.
Let the construction of any turnpike road commonly considered as among the best be properly examined; that is, let the quantity of hard-road materials that compose the crust over the subsoil be measured, and it will almost universally be found that it consists of only from three to five, or at most six inches in thickness. * Whereas, instead of this weak and defective system, it may be laid down as a general rule, that on every main road where numerous heavy waggons and stage coaches heavily laden are constantly travelling, the proper degree of strength which such a road ought to have cannot be obtained except by forming a regular foundation with large stones, set as a rough pavement, with a coating of at least six inches of broken stone of the hardest kind laid upon it; and further, that in all cases where the subsoil is elastic, it is necessary,. before the foundation is laid on, that this elastic subsoil should be rendered non-elastic by every sort of contrivance; such, for instance, amongst others, as perfect drainage, and laying a high embankment of earth upon the elastic soil, to compress it.
Although a road, if made with a thick bed of well-broken hard stones laid upon the subsoil, may, no doubt, be, to all appearance, a hard and a good
* See Mr. Telford's first Annual Report on the Holyhead Road, in 1823, where tables are given showing the result of trials made along the whole line of road from London to Shrewsbury of the depth of materials, by sinking holes into the road at short intervals. The average depth of materials was as follows on some of the trusts: —
Whetstone Trust - -4 inches.
StAlban's ditto - - 4 ditto.
Dunstable ditto - - 4J ditto.
Puddle Hill ditto - - 3| ditto. Almost all other roads, which are commonly considered good ones, would, if similar trials were made, be found to be in the same defective state.
one, still the elasticity of the subsoil will have a considerable effect in adding to the tractive force necessary to draw carriages over it; for the subsoil will yield more or less (in proportion to the elasticity which belongs to all kinds of earth,) under the incumbent weight. It is therefore only by proceeding in the way recommended, that is, by proper drainage and pressure, and by making a foundation of large stones in the form of a regular pavement, that this elasticity can be effectively diminished; for to remove it altogether is perhaps impracticable.
Rightly to understand this principle, which requires that roads should be constructed with a body or depth of materials four or five times greater than is commonly given to them, it is requisite to illustrate and establish the grounds on which it rests, by reference, first, to the laws of science concerning moving bodies, and, secondly, to experiments, which accurately prove the force of traction on different kinds of roads.
As a carriage for conveying goods or passengers when put in action becomes a moving body, in the language of science, the question to be examined and decided is, how a carriage, when once propelled, can be kept moving onwards with the least possible quantity of labour to horses, or of force of traction?
Sir Isaac Newton has laid it down as a general principle of science, that a body, when once set in motion, will continue to move uniformly forward in a straight line by its momentum, until it be