100 feet he draws 900 50 feet 810 44 feet 750 40 feet 720 30 feet 640 25 feet 540 24 feet 500 20 feet 400 10 feet 250
It is therefore seen that when the grades are 1 foot in 44 feet, or 120 feet to the mile, a horse can draw only three-fourths as much as he can on a level; where the grade is 1 foot in 24 feet, or 220 feet to the mile, he can draw only one-half as much, and on a ten per cent grade, or 520 feet to the mile, he is able to draw only one-fourth as much as on a level road.
As a chain is no stronger than its weakest link, just so the greatest load which can be hauled over a road is the load which can be hauled through the deepest mud hole or up the steepest hill on that road. The cost of haulage is, therefore, necessarily increased in proportion to the roughness of the surface or steepness of the grade. It costs one and one-half times as much to haul over a road having a five per cent grade and three times as much over one having a ten per cent grade as on a level road. As a perfectly level road can seldom be had, it is well to know the steepest allowable grade. If the hill be one of great length, it is sometimes best to have the lowest part steepest, upon which the horse is capable of exerting his full strength, and to make the slope more gentle toward the summit, to correspond with the continually decreasing strength of the fatigued animal.
So far as descent is concerned, a road should not be so steep that the wagons and carriages cannot be drawn down it with perfect ease and safety. Sir Henry Parnell considered that when the grade was no greater than one foot in thirty-five feet, vehicles could be drawn down it at a speed of twelve miles an hour with perfect safety. Gillespie says:
"It has been ascertained that a horse can for a short time double his usual exertion; also, that on the best roads he exerts a pressure against his collar of about one thirty-fifth of the load. If he can double his exertion for a time, he can pull one thirty-fifth more, and the slope which would force him to lift that proportion would be, as seen from the above table, one of one in thirty-five, or about a three per cent grade. On this slope, however, he would be compelled to double his ordinary exertion to draw a full load, and it would therefore be the maximum grade." Mr. Isaac B. Potter, an eminent authority upon roads, says: