MOVING POWERS. Tho means employed to give motion to machinery, independently of the cases in which the force of gravity is applied directly, as in turning the cylinder of is clock, are the strength of men and animals, the pressure of the atmosphere, tho expansive force of steam, and the action of wind or water. It is even possible that the recently proposed actions of the galvanic fluid and of fired gunpowder will in Vine be numbered among motive forces for impelling carriages, vessels, or machines. The first and second of the powers above named are treated under ANIMAL STRENGTH and ATMOSPHERIC RAILWAT, and the force of steam under STEAM-ENGINE.
The intensity of a moving power is always estimated by the amount of the resistance which is overcome and the apace through which the equivalent of that resistance is conveyed, or raised vertically, in a given time. Thus, in the article on ANIMAL STRENGTH, it has been shown that a man, a horse, can convey a certain weight, expressed in pounds, through a certain number of miles during a working clay; and the continued product of the weight, the distance, and the time has been made to denote the intensity of the power, one pound being the unit of weight, one mile that of distance, and one hour that of thno : in estimating, however, the power of an engine or machine, it is usual to consider one foot as the unit, of distance and one minute as the unit of time, one pound being the unit of weight ; the action of the power is, moreover, supposed to be continued during all the thuo that the machine is at work.
Originally the larger kind of engines, except such as were impelled by wind or water, were moved by the power of horses; and when other agents were employed, the gross effect of the engine was estimated by the number of hones to whose action it was equivalent ; but the intensity of horsepower is very variable, and some inconvenience was at first, on that account, experienced in estimating the relative values of engines. In order to establish, conventionally, this dynamical unit, Messrs. Boulton and Watt ascertained from trials, purposely made, that a strong horse can draw 125 lbs. at the rate of 3 miles per hour during
8 hours : therefore the measure of the power may be expressed by 3000 lbs. (=125 x 3 x 8) drawn or raised 1 mile in 8 hours; or, multi plying by 5280, the measure is 15,840,000 lbs. raised 1 foot in an equal time. This product, being divided by the number of minutes in 8 hours, gives 33,000 lbs. for the weight carried or raised 1 foot per minute continually ; and the last number is now universally adopted as a measure of the Intensity of the power of a horse. Therefore, when an engine is said to have the power of any number x of horses, it is understood that it is capable: of raising 33,000 n pounds' weight to the height of 1 foot in every minute during the continuance of its action. [Ho nse-PowEn.] The method of estimating power • by a weight carried or raised through a certain space in a certain time is capable of being applied to all engines : thus, in drawing a carriage along a road, the resistance of the carriage must be equivalent to some weight; and the reaction of the water against the paddles of a steam-vessel may always be repre sented by a certain weight which, if it were lifted by the wheel, would oppose a resistance equal to that of the water. For the useful force of steam-engines in terms of the volume of water evaporated, the pressure of the steam, the length of the stroke, &c., see STEAM-ENGINE.
Wind and water are employed as prime movers by means of the momentum arising from their velocity; and the latter, occasionally, by the pressure arising from its weight. The manner in which the force of wind is made to act in giving motion to vessels on the surface of water will be fully explained under SAII„ and in producing the revo lutions of windmill sails under WIND-SAIL; it is intended, therefore, in this place, merely to explain the method of forming an equation of equilibrium for the power of an oar in giving motion to a vessel, and to show the force of water on the paddles or float-boards of wheels which arc turned by that element.