surface of a turnpike road, the different states of repair in which different parts of it must necessarily be at any given time, but, above all, the fact that the rolling of the carriages themselves is the means by which the road is for the most part formed, consolidated and rendered smooth, make it necessary that any power of traction used upon it shall be susceptible, as occasion may require, of considerably varied energy. A newly made Macadamised road, presenting a surface of loose broken stones, offers a resistance several times greater than the same road when its surface is worn smooth. Now, as parts of every road are subject occasionally to be in this state, that relation between the power of traction and the load must be observed, which is suited to the most difficult part of the road, as well the effects of a thaw, after a severe frost to be encountered.
I have explained that the effect of incurvations on a road will obstruct the speed, whether it be a railroad or a turnpike road, but that the increased resistance offered by them on turnpike roads, bears a much smaller proportion to the resistance on the level, than is the case in railroads. The increased power, therefore, required by them, is not so great proportionally on turnpike roads as on railways; and it may be doubted, whether such increase on the regular mail-coach roads will often exceed that which is necessary to overcome the inequalities of resistance presented by the causes already explained on levels.
From the peculiar mode in which the steam-engine is used in propelling carriages, it follows that no power of traction, however intense, can be available beyond the adhesion of the impelling wheels with the surface of the road, which amount to double the weight of the carriage propelled; since that adhesion forms as it were the fulcrum or purchase by which the moving
