Eight hours per day have been proved to be the most favorable dura tion of time for the labor of both man and beast. The unit of work or labor is usually the horse-power. One horse-power is the equivalent of 33,00o foot-pounds of work done per minute or 55o foot-pounds per second. From the following data James Watt made this the standard for estimating the power of a steam-engine: If a horse walk upon level ground at the rate of two and a half miles per hour, he can continually during his working-day pull with a force of one hundred and fifty pounds. This was the work of the average horse employed in raising ore out of a mine. Now, two and a half miles per hour is a speed of zzo feet per minute, and 220 x 150 = 33,000—that is, the product of a certain number of pounds lifted vertically at the rate of a certain number of feet per min ute; hence the expression " foot-pounds," which has become the funda mental unit of work. In course of time steam-engines took the place of horses at the mines, and the naming of the power of any engine was in accordance with the number of horses whose work it did. For the sake of simplicity in having one unit, the work of men and that of animals are compared to the horse-power.
The continuous work in eight-hour periods of a man turning a crank has been fixed at fifteen pounds at a speed of 22o feet per minute, which is 220 x 15 = 3300 foot-pounds, or of a horse-power. Other applica tions of a man's hands, feet, and body doing work have been fixed at the same figure, although experimenters arrive at different results.
It must be observed that there is an essential difference between the manner of working by animals and that of working .by machines. The organic motor works wholly and most effectually by intermissions. The stress of fifteen pounds applied to the crank is not uniformly applied all the way around the circle, but by pushing and pulling the crank the ope rator, by the efforts expended upon all the quarters of the circle, has fur nished the equivalent of fifteen pounds. In treading, the action is step by step in a similar intermittent way.
and Figures t to 3 (fi/. 6r) the weight of the physiological motor is applied as the motive power simi larly to the tread-plane (fig. 4), but more completely. In the tread-wheel (fig. I) the weight of the workman is applied to the periphery of the wheel at the top, while in Figure 3 it is applied to the interior of the wheel at the bottom. In both cases the movement of the workman, laboring to ascend a "circular stair," but always retaining the same position, rotates the wheel and its shaft. In the step-wheel (Jig. 2), how
ever, he endeavors to climb a " circular ladder," but by likewise remain ing in the same place on the circumference of the wheel causes the latter, with its shaft, to revolve. The manner in which the weight of the physiological motor becomes effective with these wheels may be more clearly understood by imagining an ordinary stair placed beside the wheel. The workman ascends the stair and, stepping upon the upper step or round of the wheel, causes the latter to rotate by quietly standing on the step and sinking with it. Wheels of this description have been constructed and successfully worked by a number of workmen successively ascending the stair and sinking with the motion of the wheel. In this case, how ever, there appears a combination of muscular power and gravity.
The Horse Tread (jig. 5) is in a certain sense a horse-power with a relative movement of the motive animal to the path. While in operating the latter machine the horse walks around upon a fixed circular track, in the tread-plane the track rotates beneath the stationary animal and is secured to a shaft driving a toothed gearing. That the weight of the animal may also be made effective, the track and the shaft are some what inclined.
6 represents a portable horse-power machine enclosed by an iron casing. The transmitting shaft in the horse-track is exposed. To prevent the horses from treading upon or from stumbling over it, the shaft is covered in the track of the horses, and is connected with the horse-power by a so-called " universal joint " which admits of its being shifted into other positions. The machine appears externally as a closed casing which protects the internal gearing from rain and dust. When in use, the machine is secured to a foundation of crossed timbers. Figure 7 shows an American mounted horse-power, a machine that can be used without dismounting it from the wheels on which it is moved from place to place. It is operated by long levers which are suitably connected with the master-wheel and to which the horses are attached. The master wheel actuates a gearing which in turn drives the tumbling-rod that trans mits the power to the working machine. Figure (fit. 6o), a description of which is given on page 183, shows an inclined-plane horse-power designed for different kinds of work on the farm, more particularly as a motive power for thrashing-machines.