MECHANICAL POWERS is the name given to certain simple machines or engines, for these words are applied indifferently, either of which is occasionally used by itself in moving bodies or raising weights, or any of which are combined together in the formation of the complex constructions which are employed in manufactures and the arts. Frequently however, in investigations relating to statical equilibrium, the properties of the mechanical powers are introduced when no machine is contemplated ; as when, at some part of a plane surface, the effect of a pressure at another part is determined by the ratio of the distances from a supposed point of support.
The several machines to which the name of mechanical powers is applied are the LEVER, the WHEEL AND AXLE, the INCLINED PLANE, the WEDGE, the SCREW, and the FUNICULAR MACHINE.
The object proposed in every machine is to transmit a force from the point at which it is immediately applied to that at which some resistance is to bo overcome or some operation to be performed ; and, in the transmission, the intensity of the motive power is to be increased so that effects may be produced which could not bo accomplished by that power alone. The increase of the power is obtained by causing part of the resistance which is to be overcome to rest on the machine or on the fixed points which serve for its support, so that only the part which remains is opposed to the motive force. Thus, if it be required to raise a heavy body to a certain height from the ground, no exertion of human or animal strength may be sufficient, if directly applied, to accomplish the end; but if a plane inclined to the horizon and extend ing from the object to the spot to which tho latter is to be raised be formed, and the object can be placed on its foot, the force of gravity in the vertical direction being resolved into two forces, one of which ie destroyed by the reaction of the plane, the other may be overcome by a motive power leas in intensity than that which would be required if a direct application of fordo were made; in fact, the force which will suffice for the attainment of the end diminishes in proportion as the length of the plane is greater.
The manlier of overcoming a resistance, which is specified In this example, will serve also to illustrate the well known fact that in every application of a mechanical contrivance to overcome a resistance, as much advautage is lost in respect of time or space as is gained in respect of power. For it is evident that, iu order to raise the object
vertically throngh a space equal to the height of the plane, it would be necessary to move it over a space equal to the length of the plane ; that is, through a spa which bears the samo ratio to the vertical height as the weight of the object bears to the power required to move ft up the plane.
An account of the applications of the mechanical powers in the con struction of complex machines would involve descriptions of most 9f the engines by which human labour is abridged or dispensed with; some of the most typical of these are referred to under Meenreee, where the reader will also find a reference to works of authority on the subject.
In determining the efficacy of the mechanical powers it is evidently necessary to consider their parts as mathematical lines, to assume that the axles are without friction and cords without rigidity, so that a perfect equilibrium may subsist in the machine itself before the moving power is applied. The most simple lever, for example, of a physical kind, is a rod of wood or a bar of iron, the arms of which, on opposite sides of the fulcrum, are of unequal weights ; and, in order to reduce such a lever to a state from which the exact relation between the opposing powers may be found, the weight of each arm must be com puted, and being, in imagination, applied at the centre of gravity of the arm, the product of the weight multiplied by the distance of the centre of gravity from the fulcrum is to be added to the momentum of the weight actually applied to the same arm of the lever. The sums of these momenta, when the actual machine is in equilibrio, serve to determine the correct relation between the opposing powers. The conditions of equilibrium being determined, any excess of the motive power above that which enters into those conditions will evidently overcome the resistance, or produce motion in the machine.