The results of such an investigaticin should be reduced to a plan and section ; the plan of the road being on a scale not less than 66 yards to an inch, and the section not less than 30 feet to an inch.
ties, render it especially necessary that a proper limitation should be imposed upon the inclinations or acclivities on every line of road on which much traffic is car ried on. As, however, this reduction of hills in a country where much of surface exists is attended with a consid erable outlay of capital, the engineer will have to balance the cost of constructing a road, having the best possible inclina tions against the advantages to be ob tained in the permanent working of the road ; and if the expected traffic be not such as to yield advantages proportionate to the capital absorbed, greater rates of inclination must be allowed to the hills, with a view to diminish the extent of the works, and to render the expense of con structing the road proportionate to the traffic expected upon it.
A dead level, even where it can bo ob taMed, is not the best course for a road ; a certain inclination of the surface facili tates the drainage, and keeps the road in a dry state. There is a certain inclina tion, depending on the degree of perfec-* tion given to the surface of the road, and on the structure of the carriages worked upon it, which cannot be exceeded with out .a direct loss of tractive power ; this inclination or acclivity is that, in descend ing which, at a uniform speed, the traces slacken, or which causes the carriages to press on the horses : the limiting incli nation within which this effect does not take place is called the angle of repose.
On all acclivities less steep than the angle of repose, a certain amount of trac tive' force is necessary in the descent as well as in the ascent ; and the mean of the two drawing forces, ascending and descending, is equal to the force along a level road. Thus, on such acclivities as much power is gained in the descent as is lost in the ascent ; but on acclivities which are more steep than the angle of repose, the load presses on the horses during their descent, so as to impede their action, and their power is expended in checking the descent of the load : or, if this effect be prevented by the use of any form of drag or break, then the power expended on such drag or break corre sponds to an equal quantity of mechani cal power expended in the ascent, for which no equivalent is obtained in the descent.
On well-constructed roads, with car riages such as now are generally used in England, the angle of repose may be taken at about one in thirty-six ; and this is consequently an acclivity which ought not to be exceeded on roads over which much traffic is carried.
The expedients by which the requisite inclinations are obtained on common roads are the same as those which are re sorted to in the construction of railways.
(See RAILROADS.) The exact course of the road, and the degree of its acclivities being determined, the next thing to be considered is the formation of its surface. The qualities which ought to be imparted to it are two fidd—first, it should be smooth ; second ly, it should be hard ; and the goodness oftlie road will be exactly in the propor tion of the degree in which these quali ties can he imparted to it, and perma nently maintained upon it. An error pre vailed among road engineers until is very recent period. It was considered that smoothness of surface alone was sufficient for the perfection of a road; and that, provded it could be made sufficiently durable, it was unimportant how soft or yielding the coating of the road might be. This error, into which, among others, Macadam himself fell, was based upon a neglect of one of the most important circumstances to be considered in the construction of a road. The main object to be attained by all roads is the diminu tion of the resistance which a carriage opposes to the tractive power. Other things being the same, it was sufficiently apparent that this resistance would be diminished by increasing the smoothness of the road surface. But roughness or unevenness of surface is not the only cause of resistance to the tractive power ; if two roads have their surfaces equally smooth and even, bet one is soft and elastic, so as to yield tinder the pressure of the wheel, recovering its form as the wheel advances, and the other is hard and unyielding, the resistance to the tractive power will be greater on the soft and yielding road than on the hard and unyielding road; and this augmentation of resistance will he in proportion to the softness of the surface. That this would be the case, admits of immediate demon stration on mechanical and mathematical principles ; but, without resorting to these, it must be sufficiently apparent from the results of the most common ex perience. A surf:tee of velvet may be as smooth and even as a surface of ice ; but if an ivory ball be rolled on the latter, it will continue its motion munch longer than on the former. In fact, the wheels of a carriage in passing along a soft road sink into its surface, as the ball would sink into the pile of the velvet; and although in virtue of its elasticity, the surface of the road, like that of the velvet, may re cover its smoothness after the pressure has been removed from it, still a resist ance will be offered to the drawing or im pelling power, which would not be pro a hard and unyielding surface equally smooth.