| Asphalted roadway | 1·0 | ||||
| Paved | roadway, | dry and in good order | 1·5 to 2·0 | ||
| „ | „ | in fair order | 2·0 „ 2·5 | ||
| „ | „ | but covered with mud | 2·0 „ 2·7 | ||
| Macadamised | roadway, | dry and in good order | 2·5 „ 3·0 | ||
| „ | „ | in a wet state | 3·3 | ||
| „ | „ | in fair order | 4·5 | ||
| „ | „ | but covered with mud | 5·5 | ||
| „ | „ | with the stones loose | 5·0 „ 8·2 | ||
There are four forces constantly at work tending to destroy the momentum of vehicles passing along a roadway: they are gravity, collision, friction, and the resistance of the air.
The first of these is lessened by easy gradients in a road, the second can be overcome to a great extent by evenness of surface, the third by hardness, and the fourth, as well as all the others, by giving sufficient foothold to the animal drawing the vehicle.
Another excellent table[9] prepared from experiments made by Mr. Amos on different descriptions of pavement in the City of London may be useful, and is here given:—
| Road Material. | Speed in Miles per hour. | Draught in lbs. | Fraction of Load. | Tractive Force in Decimals of the Load. | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Gravelly Macadam in a side street | - | 6 | ·945 | 126 | ·6 | 145·3 | ·0219 | |||
| 3 | ·45 | 114 | ·322 | 150·3 | ·0197 | |||||
| Granite pitching by side of tramway | - | 5 | ·15 | 70 | ·963 | 181·1 | ·0123 | |||
| 3 | ·196 | 41 | ·932 | 1137·3 | ·0072 | |||||
| 2 | ·557 | 47 | ·572 | 1121 | ·0082 | |||||
| Granite Macadam “freshly laid” | - | 4 | ·239 | 262 | ·886 | 121·9 | ·0456 | |||
| 2 | ·775 | 242 | ·726 | 123·7 | ·0421 | |||||
| Asphalte Pavement | - | 5 | ·025 | 91 | ·525 | 164·9 | ·0158 | |||
| 3 | ·56 | 69 | ·753 | 182·5 | ·0121 | |||||
| 5 | ·687 | 84 | ·268 | 168·3 | ·0111 | |||||
| Wood Pavement | - | 3 | ·932 | 118 | ·163 | 148·7 | ·0205 | |||
| 3 | ·278 | 102 | ·412 | 156·2 | ·0177 | |||||
| 3 | ·827 | 100 | ·066 | 157·5 | ·0173 | |||||
| Macadam road, very good on Victoria Embankment | - | 6 | ·65 | 109 | ·06 | 152·7 | ·0181 | |||
The following table from Law’s ‘Rudimentary Treatise on Civil Engineering’ shows the force required to move a load of a ton weight on different descriptions of roadway, the limiting angle of resistance, and the greatest inclination which should be given to the road being also stated.
| Description of the Road. | Force in lbs. required to move a ton. | Limiting angle of resistance. | Greatest inclination which should be given to the road. | |||
|---|---|---|---|---|---|---|
| ° | ′ | |||||
| Well laid pavement | 33 | 0 | 50 | 1 | in | 68 |
| Broken stone surface on a bottom of rough pavement or concrete | 46 | 1 | 11 | 1 | „ | 49 |
| Broken stone surface laid on an old flint road | 65 | 1 | 40 | 1 | „ | 34 |
| Gravel road | 147 | 3 | 45 | 1 | „ | 15 |
As a matter of fact, however, the gradient of a macadamised road should not, if possible, exceed 1 in 20,[10] experience having shown that a horse, unless the hill is a very long one, is able to draw his ordinary load for a level up such an inclination, whereas, if it is steeper he is sometimes stopped altogether, even though the carter tries the zigzag route so as to obtain an artificial ease of gradient.
The table given in ‘Molesworth’ upon the same subject is too well known to be repeated, and another table may be found in Sir Henry Parnell’s work on roads, which gives a comparison between the draught necessary on a well-paved road at 2, on a well-made, clean macadamised road at 5, whereas on a wet and muddy gravel or flint road it rises to 32!
Mr. T. D. Hope, of Liverpool, assuming the power of traction at 100, gives the following table:—