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 the subsoil, may, no doubt, be, to all appearance, a hard and a good

upon

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:

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.

F

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

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, 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, 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

*See Wood's Mechanics, p. 20.

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.” *

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 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 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

* Elements of Natural Philosophy.