4th. Upon hard macadamized, and upon paved roads, the traction increases with the velocity: the increments of traction being directly proportional to the increments of velocity above the velocity 3·28 feet per second, or about 2¼ miles per hour. The equal increment of traction thus due to each equal increment of velocity is less as the road is more smooth, and the carriage less rigid or better hung.
5th. Upon soft roads of earth, or sand, or turf, or roads fresh and thickly gravelled, the traction is independent of the velocity.
6th. Upon a well-made and compact pavement of hewn stones, the traction at a walking pace is not more than three-fourths of that upon the best macadamized roads under similar circumstances; at a trotting pace it is equal to it.
7th. The destruction of the road is in all cases greater, as the diameters of the wheels are less, and it is greater in carriages without than with springs.
In Sir H. Parnell’s book on roads, p. 73, we are told that Sir John Macneill, by means of an instrument invented by himself for measuring the tractive force required on different kinds of road, obtained the following general results as to the power requisite to move a ton weight under ordinary circumstances, at a very low velocity.
| Description of Road. | Force, in pounds, required to move a ton. |
|---|---|
| On a well-made pavement | 33 |
| On a road made with six inches of broken stone of great hardness, laid either on a foundation of large stones, set in the form of a pavement, or upon a bottoming of concrete | 46 |
| On an old flint road, or a road made with a thick coating of broken stone, laid on earth | 65 |
| On a road made with a thick coating of gravel, laid on earth | 147 |
In the same work the relative degrees of resistance to traction on the several kinds of roads are thus expressed:—
| On a timber surface | 2 |
| On a paved road | 2 |
| On a well-made broken stone road, in a dry clean state | 5 |
| On a well-made broken stone road, covered with dust | 8 |
| On a well-made broken stone road, wet and muddy | 10 |
| On a gravel or flint road, in a dry clean state | 13 |
| On a gravel or flint road, in a wet muddy state | 32 |