Table 10, page 34, is computed from equation (3) for a value of t equal to one tenth of the weight of the horse. The top line of the table shows the loads that a horse can draw on the level on the various road surfaces; and any column of the table shows the load that a horse can pull on any grade for that particular road surface.
As showing the different effects of grades upon different roads, notice that on a muddy earth road a 1 per cent grade reduces the load less than one tenth, while on asphalt a 1 per cent grade reduces the load more than one half; or, again, notice that with a 5 per cent grade, on iron rails the load is less than one twentieth of the load on the level, while on the best earth road the load is one fifth of that on the level.
Table 10 shows the load a horse can draw upon different grades and different road surfaces when exerting a uniform pull equal to one tenth of its weight. If we desire to know the maximum load which a horse can draw up any grade, we must insert in equation (3) the maximum value of t and compute the corresponding value of L. The value of t to be used in this computation will depend upon the length of the grade and upon the frequency with which it occurs. If the grade is only a few hundred feet long, it
will probably be safe to assume a pull equal to one fourth of the weight of the horse; but this value should be assumed only for the maximum grade, since such pulling is too exhausting for continuous work.
Table 11, page 34, presents the same data as Table 10, but in a slightly different form.
The maximum load which a horse can draw upon any road, particularly upon the steepest grade, is not, however, necessarily proportional to the rate of grade and to the resistance, since the pull that a horse can exert depends upon the foothold. Owing to the danger of slipping on steep grades, particularly when the road is wet or icy, it is not customary to lay sheet asphalt on grades of more than 4 per cent, or ordinary stone blocks on more than 10 per cent. On steeper grades, special forms of stone blocks are sometimes employed to increase the tractive power by affording better foothold for the horses.