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PRINCIPLES OF ROAD-MAKING.
In this chapter, the general principles will be examined, according to which the art of constructing a road should be practised ; and the particular methods will be explained, by which various kinds of roads should be constructed. The art of road-making, like every other art, must essentially depend for its being successfully conducted on its being exercised in conformity with certain general principles, and the justness of these principles should be rendered so clear and self-evident as not to admit of any controversy.*
* “A knowledge of true principles is indispensably necessary in every art, and in that of making roads as much as in any other. Some preliminary species of knowledge is very necessary in every superintendant or surveyor. A beaten track of knowledge is but a bad guide in cases which very frequently occur, when, amongst several ways, the best is to be preferred.”—M. S. Haldimand, Secretary to the Bailiwick of Yverdun, on the Construction of Highways.
One of the most important and most obviously correct of these principles, is that which requires a road to be made of such a degree of substance, as shall be in a due proportion to the weight and number of the carriages that are to travel over it.
But although this is, in appearance, a selfevident proposition, in practice no rule is so universally violated.
Let the construction of any turnpike road, of one commonly considered as among the best, be properly examined ; that is, let measure be taken of the quantity of hard-road materials that compose the crust of the road over the subsoil, and it will almost universally be found that it consists of only from three to five, or six inches in thickness.* Whereas, instead of this weak and defective system of road
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.
making, it may be laid down as a general rule, that on every main road where numerous heavy waggons and heavy loaded stage coaches are constantly travelling, the proper degree of strength which such a road ought to have cannot be obtained except by forming a regular foundation constructed 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, amongst others, for instance, as perfect drainage, and laying a high embankment of earth upon the elastic soil, to compress it.
The right understanding of this principle of road-making, which requires roads to be constructed with four or five times a greater body or depth of materials than is commonly given to them, is of such great importance, that it is requisite to illustrate and establish the grounds on which it rests; first, by reference to the laws of science concerning moving bodies, and secondly, by reference 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 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 kinds 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 1st, 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,
• See Wood's Mechanics, p. 20.
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 a 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 substrata of the road.” *
An ivory ball, set in motion with a certain velocity over a Turkey carpet, will suffer a visible relaxation of its course ; but, with the same impelling force, it will advance further if rolled over a superfine cloth; still further over
Elements of Natural Philosophy.