TRACTION, in Mechanics, is the act of drawing a body along a plane, usually by the power of men, animals, or steam ; as when a vessel is towed on the surface of water or a carriage moved upon a road. The power exerted in order to produce the effect is called the force of traction.
Numerous experiments have been made for the purpose of ascer taining the value of a force so exerted; and when men are employed to draw laden boats on canals, it is found that if the work be continued for several days successively, of eight hours each, the force of traction is equivalent to a weight of 31i lbs., moved at the rate of two feet per second, or mile per hour (it being understood that such weight is imagined to be raised vertically by means of a rope passing over a pulley, and drawn in a horizontal direction). The force of traction exerted when, without moving from his place, a man pulls horizontally against a weight so suspended, is estimated at 70 lbs. The action of a horse in drawing a vessel on a canal is said to be equivalent to a weight of 180 lbs. raised vertically, as above supposed, with a velocity of 3i feet per second, or 21 miles per hour ; but this estimate has been considered too high ; and from experiments which have been made on the power of horses in waggons, carts, and coaches, on level ground, it is found that the force of traction exerted by a stout horse is equivalent to 80 lbs. raised at the rate of 41 feet per second, or 3 miles per hour. Mr. Tredgold considers that a horse exerts a force of traction expressed by 125 lbs. raised at the rate of 3i feet per second, or 2] miles per hour. A man or a horse can however double his power of traction for a few minutes without being injured by the exertion ; and when the carriage is in motion, so that the friction on the ground is alone to be over come, a horse can draw, during a short time, on a level road, a weight exceeding 1500 lbs.
The force of traction is found to vary nearly with the term where to is the greatest walking velocity of a man or horse when unresisted (6 feet per second, or 4 miles per hour, for a man ; and 10 feet per second, or 6; miles per hour, for a horse), and v is the velocity with which the vessel or carriage is moved. From theoretical con siderations it has been determined that tho greatest effect is produced wheri the velocity of the object moved is one-third of that with which the roan or animal can walk when unresisted.
If a wheel-carriage were situated on a level plane which opposed no resistance, it is evident that, whatever were the diameter of the wheels, the smallest conceivable power of traction applied to the axle would suffice to put the carriage in motion. But when a wheel in moving meets with an obstacle on the ground, that obstacle is pressed at the point of contact by a force acting in the direction of a line drawn to it from the centre of the wheel, and arising from that part of the weight which is supported by the wheel, together with the force of traction ; therefore by the " resolution of forces," the ratio between the resistance which is to be overcome by the moving-power and the weight on the wheel will become less as the diameter of the wheel is increased : also the most advantageous direction in which the force of traction can be exerted is perpendicular to the line of pressure drawn from the centre of the -wheel to the obstacle. But the height of the wheels cannot
exceed certain limits depending on the use to which the carriage is applied ; and when the latter has four wheels, the height of those which are in front must be such only as will allow it to be turned round within a given apace; also, when a horse is employed to move a carriage, attention must be paid to the conditions under which his power may be advantageously exerted.
It was first observed by M. Deparcieux, and published in the WI/mires do l'Aeaddinie des Sciences, 1760, that horses draw heavy loads rather by their weight than by their muscular force. Dr. (Sir David) Brewster has also remarked that when the resistance is great a horse lifts both its fore-feet from the ground ; then, using his hinder feet as a fulcrum, he allows his body to descend its weight, and Oita overcomes the obstacle : and it may be that when this action takes place with a two-wheeled carriage, if the loading is dis posed so that some portion of it may press on the horse's back, the effect of the animal's weight will thereby be increased. Now if the traces, or the shafts of the carriage, were attached to the horse's collar near his centre of gravity, a liue imagined to be drawn from the latter point to his hinder-feet may represent his weight, and a lino drawn perpendicularly from his feet upon a plane passing through the traces or shafts may represent the lever of resistance : but while the former line remains the same, this lever becomes less as the plane of traction (that of the traces or shafts) inclines more upwards from the wheel ; and therefore, in order that the power of the horse may be advan tageously applied, the diameter of the wheel should be as small as is consistent with other circumstances.
Experiments have shown that when the angle of traction, as it is called, that is, the angle which the plane of the traces makes with the road on which the carriage is moving, is 15 or 16 degrees, a horse pulls with good effect: and the height of the points at which the traces are attached to a horse's collar being about 4 feet G inches from tho ground, it follows that, in order to obtain this inclination, the lower extremities of the traces or shafts should be 2 feet 3 inches from the ground. In general however, in two-wheeled carriages, the height of these extremities is about 3 feet.
As an example of the force of traction exerted by steam, it may be stated that on a level line of railway, an engine with an 11-inch cylinder, and bovine an effective pressure of 50 lbs. per square inch in the boiler, drew 50 tons at the rate of 30 miles per hour, working 10 hours and that the same engine, with an equal pressure in the boiler, drew 160 tons at the rate of I5i miles per hour. (Tambour On Locomotive Engines.') The resistances to be overcome, or, in other words, the efforts of traction required upon the various systems of intercommunication, may be stated as follows :— . .