POWER OF A HORSE. The horizontal pull which a horse can exert depends upon its weight, its form or build, the method of hitching, the foothold afforded by the surface, the speed, the length of duration of the effort, the rest-time between efforts, and the tax upon the future efficiency of the horse. The chief of these are the weight, the speed, and the length of the effort.
Horses vary in weight from 800 to 1,800 pounds. The larger horses do not usually travel more than 2f or 3 miles per hour. With reasonably good footing a horse can exert a pull equal to one tenth of his weight at a speed of 2f miles per hour (34 feet per second) for 10 hours per day for 6 days per week and keep in condition. This is a common rate of exertion by farm horses in pulling plows, mowers, and other agricultural implements. On this basis a 1250 pound horse would develop 550 foot-pounds per second (the con ventional horse-power), and 16,500,000 foot-pounds' per day. A lighter horse will exert a proportionally less force. This may be considered about the limit of endurance. If the time of the effort is decreased, the draft may be proportionally increased; or if the speed is increased, the draft must be decreased in a like proportion. In other words, the foot-pounds of energy that can be developed per day by any particular horse is practically constant.
The maximum draft for a horse is about half of his weight, although horses have been known to exert a pull of two thirds of their weight. Most horses can exert a tractive power equal to half their weight, at a slow walk for about 100 feet. On the road in emergencies, a& in starting the load or in overcoming obstacles, a horse may be expected to exert a pull equal to half his weight, but at this rate he would develop a day's energy in about 2 hours; and consequently if he is expected to work all day, he should not be called upon to exert his maximum power except for a short time. Similarly, a horse can exert a draft equal to one quarter of his weight for a longer time. The working tractive power of a horse may be taken as one tenth of its weight, with an ordinary maximum of one quarter, and in great emergencies a maximum of one half its weight.
Increasing the number of horses does not increase the power proportionally—for somewhat obvious reasons. It is stated that for a two-horse team the efficiency of each horse is about 95 per cent; for a three horse team, about 85 per cent. Of course such data are not much more than guesses.
Equation (1) is not mathematically correct, since it assumes that the weight of the horse is always normal to the road surface. However, the formula is sufficiently accurate for use in comparing the relative tractive power of a horse on different grades (see 41). At best such a formula can be only approximate, since the tractive power varies greatly with the foothold.