Whether the road be constructed of earth, stone, or gravel, steep grades should always be avoided if possible. They become covered at times with coatings of ice or slippery soil, making them very difficult to ascend with loaded vehicles, as well as dangerous to descend. They allow water to rush down at such a rate as to wash great gaps alongside or to carry the surfacing material away. As the grade increases in steepness either the load has to be diminished in proportion or more horses or power attached. From Gillespie we find that if a horse can draw on a level one thousand pounds, on a rise of-- 1 foot in-- Pounds 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: