ENGINE, HYDRAULICS, PNEUMATICS, &C. &c.
The modes of applying mechanical forces are almost as various as the ma chines that are constructed, and the pur poses for which they are employed. In general the human strength is applied by means of levers, or winches, or by walk ing wheels, which slide beneath them as they attempt to ascend. The force of other animals is applied by a horizontal arm projecting from a vertical axis, to which they are harnessed. When mo tion is simply communicated to a sub stance placed before the moving body, such materials are used as are capable of exerting a repulsive force ; but when the body to be moved is behind the moving power, and is pulled along with it, chains or ropes are sometimes more convenient. When the direction of motion communi cated is also to be changed, levers or cranks may be employed, united by joints or hinges of various kinds. Sometimes a long series of connected rods is suspend ed by other rods or chains, so as to con vey the effect of the force to a considera ble distance; in this case the motion is generally alternate, as when pumps are worked by means of a water-wheel at a distance from the shafts in which the pumps are placed. For the communica tion of a rotatory motion, Dr, Hooke's universal joint, formed by a cross, mak ing the diameters of two semicircles, one of which is fixed at the end of each axis, is frequently used. The best mode of connecting a rotatory motion with an al. ternate one is, in all common cases, to employ a crank, acting on one end of a long rod which has a joint at the other.
If the rotatory motion of the crank be equable, the progressive motion of the rod will be gradually accelerated and re tarded, and for a considerable part of the revolution the force exerted will be near ly uniform, The force applied to a ma chine may, in general, be divided into two portions, the one employed in opposing another force, so as to produce equili brium only, the other in generating mo mentum. With respect to the first por tion, a single crank has the inconvenience of changing continually the mechanical advantage of the machinery ; with regard to the second, its motion in the second quarter of its revolution is accelerated, instead of being retarded, by the inertia; which this portion of the force is intended to overcome ; hence the motion is irregu lar. This difficulty may be remedied by employing cranks in pairs, one of which being fixed so as to make a right angle with the other, which is moreover the best position for two winches to be turned by two since the point of the circle, in which a man can exert his greatest strength, is nearly at the dis tance of a right angle, or a little more, from the point at which his torte is smallest. But of all the modes of com municating motion, the most extensively useful is employment of wheel-work, which is capable of varying its direction and its velocity without any limit. See WHEEL•worlc.