will sustain a part of the weight inversely proportional to the number of similar portions which the surface contains. Let us now suppose that a cart, carrying a load of 16 hundred weight, is supported upon wheels whose rims are 4 inches in breadth, and that one of the wheels passes over 4 stones, each of them an inch broad, and equally high, and capable of being pulverized only by a pressure of 400 weight. Then, as each wheel sustains one half of the load, and as the wheel which passes over the stones has 4 points of support, each stone will bear a weight of 200 weight, and therefore will not be broken. But if the same cart, with rims only 2 inches in breadth, should pass the same way, it will cover only 2 of the stones; and the wheel having now only two points of support, each stone will be pressed with a weight of 400 weight, and will therefore be reduced to powder. Hence we may infer, that narrow wheels are, in another point of view, injurious to the roads, by pulverizing the materials of which they are composed. As the rims of wheels wear soonest at their Practical edges, they should be made thinner in the mid- remarks. dle, and ought to be fastened to the fellies with nails of such a kind, that their heads may not rise above the surface of the rim. In some military waggons, we have seen the heads of these nails rising an inch above the rims, which not only destroys the pavements of streets, but opposes a continual resistance to the motion of the wheel. If these nails were 8 in number, the wheel would experience the same resistance as if it had to surmount 8 obstacles, 1 inch high, during every revolution. The fellies on which the rims are fixed should, in carriages, be 3 inches deep, and in waggons 4 inches. The naves Position of the wheels. tages of trces. should be thickest at the place where the spokes On the Position of the Wheels. It must naturally occur to every person reflecting upon this subject, that the axle-trees should be straight, and the wheels perfectly parallel, so that they may not be wider at their highest than at their lowest point, whether they are of a conical or a cylindrical form. In this country, however, the wheels are always made concave, and Disadvan- the ends of the axle-trees are universally bent bent axle- downwards, in order to make them spread at the top and approach nearer below. In some carriages which we have examined, where the wheels were only 4 feet 6 inches in diameter, the distance of the wheels at top was fully 6 feet, and their distance below only 4 feet 8 inches. By this foolish practice, the very advantages which may be derived from the concavity of the wheels are completely taken away, while many of the disadvantages remain; more room is taken up in the coach-house, and the carriage is more liable to be overturned, by the contraction of its base. With some mechanics it is a practice to bend the ends of the axle-trees forwards, and thus make the wheels wider behind than before. This blunder has been strenuously defended by Mr. Henry Beighton, who maintains that wheels in this position are more favourable for turning, since, when the wheels are parallel, the outermost would press against the linch-pin, and the innermost would rub against the shoulder of the axle-tree. In rectilineal motions, however, these converging wheels engender a great deal of friction, both on the axle and the ground, and must therefore be more disadvantageous than parallel ones. This, indeed, is allowed by Mr. Beighton; but he seems to found his opinion upon this principle, that as the roads are seldom straight lines, the wheels should be more adapted for curvilineal than for rectilineal motion. In what part of the world Mr. Beighton has examined the roads we cannot say ; but of this we are sure, that there are no such flexures in the roads of Scotland. On the Line of Traction, and the Method by which Horses exert their Strength. M. Camus, a gentleman of Lorrain, was the Line of first person who treated on the line of traction. 8 traction. He attempted to shew that it should be a horizontal line, or rather that it should always be parallel to the ground on which the carriage is moving, both because the horse can exert his greatest strength in this direction, and because the line of draught being perpendicular to the vertical spoke of the wheel, acts with the largest possible lever. M. Couplet, however, considering that the roads are never perfectly level, 8 Traite des Forces Mouvantes, p. 387. 9 Reflexions, sur le tirage des charrettes, Mem. de P'Acad. Paris, 1733, 8vo, pp. 75, 86. Fig. 5. and that the wheels are constantly surmounting small eminences, even in the best roads, recommends the line of traction to be oblique to the PLATE II, horizon. By this means the line of draught HA, (which is by far too much inclined in the figure), will in general be perpendicular to the lever AC which mounts the eminence, and will therefore act with the longest lever when there is the greatest necessity for it. We ought to consider also, that when a horse pulls hard against any load, he always brings his breast nearer the ground, and therefore it follows, that if a horizontal line of traction is preferable to all others, the direction of the traces should be inclined to the horizon when the horse is at rest, in order that it may be horizontal when he lowers his breast and exerts his utmost force. How horses exert their The particular manner, however, in which livstrength. ing agents exert their strength against great loads, seems to have been unknown both to Camus and Couplet, and to many succeeding writers upon this subject. It is to M. Deparcieux, an excellent philosopher and ingenious mechanic, that we are indebted for the only accurate information with which we are furnished; and we are sorry to see, that philosophers who flourished after him have overlooked his important instructions. In his Memoir on the draught of horses,' he has shewn, in the most satisfactory manner, that animals draw by their weight, and not by the force of their muscles. In four-footed animals, the hinder feet is the fulcrum of the lever by which their weight acts against the load, and Sur le Tirage des Chevaux, published in the Mem. de l'Acad. Paris, 1760, 4, p. 263, 8, p. 275. when the animal pulls hard, it depresses its chest, and thus increases the lever of its weight, and diminishes the lever by which the load resists its efforts. Thus, let P be the load, DA the PLATZ II, line of traction, and let us suppose FC to be Fig. 5. the hinder leg of the horse, AF part of its body, A its chest or centre of gravity, and CE the level road. Then AFC will represent the crooked lever by which the horse acts, which is equivalent to the straight one AC. But when the horse's weight acts downwards at A, round Cas a centre, so as to drag forward the rope AD, and raise the load P, CE will represent the power of the lever in this position, or the lever of the horse's weight, and CF the lever by which it is resisted by the load, or the lever of resistance. Now, if the horse lowers its centre of gravity A, which it always does when it pulls hard, it is evident that CE, the lever of its weight, will be increased, while CF, the lever of its resistance, will be diminished, for the line of traction AD will approach nearer to CE. Hence we see the great benefit which may be derived from large horses, for the lever AC necessarily increases with their size, and their power is always proportioned to the length of this lever, their weight remaining the same. Large horses, therefore, and other animals, will draw more than small ones, even though they have less muscular force, and are unable to carry such a heavy burden. The force of the muscles tends only to 2 It may be imagined that the fore feet of the horse prevent it from acting in this manner; but Deparcieux has shewn by experiment that the fore feet bear a much less part of the horse's weight when he draws than when he is at rest. |