MOVING POWERS. The 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 a clock,) are chiefly the following :—the strength of men and animals, tho pressure of the atmo sphere, the expansive force of steam, and the action of wind or water. It is even probable that the recently proposed actions of the gal vanic fluid and of fired gunpowder or gun cotton, will in time be numbered among mo tive forces for impelling carriages, vessels, or machines.
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 horses to whose action it was equivalent.. A strong horse being able to draw 125 lbs. at the rate of 3 miles per hour during 8 hours; the product of these numbers, multiplied by the number of feet in a mile, and divided by the number of minutes in 8 hours, gives 33,000 lbs. for the weight carried or raised 1 foot per minute continually. This last num ber is the usual representation of the power of a horse ; but the ' horse power' of engi neers' estimates is felt to be a vague standard, 'la ld will probably give way to something more scientific. The force of steam-engines is esti mated according to the volume of water eva porated, the pressure of the steam, the length of the stroke, &c.
Wind and Water are employed as prime movers by means of the momentum arising from their velocity ; and the latter, occasion ally, by the pressure arising from its weight.
The force of wind is made to act on sails in giving motion to vessels on the surface of water, and also in producing the revolutions - of windmill sails. The velocity of a vessel impelled by steam, in which paddlewheels are employed, might easily be found if it were possible to determine, nearly, the number of square feet of paddle which, on both sides of the vessel, are at every moment acting effi ciently against the water. To determine the exact amount of moving power available at any given time and place, and under any given circumstances, is one of the most important duties of the engineer; since upon it must depend the power of his machinery.
In 1850 Mr. Cottam patented an apparatus for ascertaining the amount of motive power employed in 'working machinery. It is com posed of a plate, to which a circular spring is attached by one end, while the other end is made fast to a lever. The lever is also at tached to a pointer which moves over a scale, whereby the power exerted will be indicated. The pointer also carries a pencil, which tra vels over the top of one of a pair of cylinders, round which a roll of paper is wound. The curve described on the roll of paper indicate the force applied. The cylinders are made to revolve by toothed gearing.