MACHINES

—The parts of a machine are of two kinds: (a) fixed parts, or frame; (6) moving parts, or mechanism. The design of the frame depends on the size and formation of the moving parts or mechanism. In a machine, energy is communicated to the moving parts by prime movers, or mechanism, and thence to the working parts. For the purposes of scientific design, the usual method is to consider first the force and the variation of force required; to proceed then to a solution of the motion to be given by the machinery; and, lastly, to design the machine for that particular motion. _ All varieties of machinery are based upon the three simple mechanical powers: Gi) the lever; (2) the pulley; (3) the inclined plane. A lever is the name given to any rod paken as absolutely rigid and inflexible) capable of motion about a fixed point or support (fulcrum), and under the influence of two forces and the reaction at the fulcrum. Since the Durpose of machines is to Overcome resistance or weight, one force resists Motion and the other produces it, Levers may be straight or bent, but in either case the power, P (the force exerted), the weight, W (the force overcome), and the fulcrum, F, may be arranged in three different ways, whence the usual division of levers into the first, second, and third orders. The arrangement wend Re peprernted thus: Ww F P FB In all, however, the general principle of the lever is satisfied—viz., that the power and weight vary inversely as their distances from the fulcrum. This may be expre ed for equilibrium, thus: Generally, therefore, the inference holds good that the smaller the power the greater the distance it must move through, and that a gain in power is, hence, a lo of speed. In the second and third orders it is always speed which is gained, power being lost—lf. (t.e.,)at a disadvantage. The first order of levers includes balances, see-saws, sci ors, and spades in the act of raising earth; the second, crowbars and wheelbarrows; and the third, sugar-tongs and fishing-rods. A pulley (which, like the wheel and axle, is equivalent to a cortinuous lever) is simply a cylinder with a groove cut on its circumference to carry a rope; it is capable of rotation about an axis carried in a piece called a block. The figure illustrates a movable pulley A and a fixed pulley B. Suppose a_ weight carried at 6. Then the whole weight (W) is carried by the portions of the cord ad and cd, the tension in each being = Ee If the cords are not parallel the forces must be resolved, and the tension in the cord being greater there is a lo of power. A movable pulley comprises a number of wheels in a block: this apparatus is called a block and fall. A combination of pulleys in one machine is called a system. The most common combination is a block and tackle, consisting of two blocks containing pulleys. The upper is fixed, the lower carries the weight. The rope is fastened to one or other of the blocks, and pa es round the sheaves or wheels. The inclined plane is a device for the lifting of weights. If a body rests on a horizontal plane, the plane sustains the whole weight. If the plane be inclined, however, only a part of the weight is carried by the plane; hence the use of this device. The wheel and axle is a wheel provided with a cylindrical axle, both wheel and axle being fitted to take a rope round the circumference. — When it is required to lift a weight, this is attached to the axle rope, and the power is then applied to the wheel rope, the power supporting a larger weight in proportion to the diameters of the wheel and axle.