MECHANICAL MOVEMENTS.
Mechanical movements constitute an essential part of transmitting machinery; they may be called the organs of transmission, and they therefore take form in accordance with the direction of the intended motion, and are proportioned to meet the demands of the forces which they are devised to transmit. They may be either simple or complex. When a body or any piece of mechanism moves in a straight line it has a rectilinear motion; when it moves in a curved line, a curvilinear motion. When a body moves constantly in the same direction it has a continuous motion; if it move backward and forward it has a recipro caling motion. There are motions as well as motions. If a body move without variation over equal spaces in equal intervals of time, it has a uniform motion; if it move over unequal spaces in equal intervals of time, it has a vari able motion.
While it is certain that no machine can be constructed without employ ing mechanical movements or devices in one shape or another, it is demon strable that no power is gained by their use, in whatever way they may be combined or operated, as the force that is applied at one point can only be exerted at some other point, and this force is always diminished by friction and other incidental causes. The action of force in a machine at equilib rium, considered without reference to frictions and the weight of the machine itself, may be thus formulated: The power multiplied by the distance through which it moves in a vertical direction is equal to the weight multiplied by the distance through which it moves in a vertical direction. This is called the "golden rule" of mechanics. The power performance of every machine may be reduced to the expression: "What is gained in power is lost in speed;" or, conversely: Whatever is gained in the rapidity of execution is compensated by the necessity of exerting additional force. The function of mechanism is to receive, concentrate, diffuse, and apply power to overcome resistance. The combinations of mechanism are numberless, but the primary elements are only two— namely, the lever and the inclined plane. By the lever, power is trans
mitted by circular or angular action—that is, by action about an axis; by the inclined plane, power is transmitted by rectilinear as well as by circular action. The principle of the lever is the basis of the pulley and of the wheel and axle; that of the inclined plane is the basis of the wedge and of the screw.
Mechanical movements, whether simple or complex, must rest or move upon fixed bases or a framework shaped and adapted to their needs. A train of mechanism must be so mounted that when the receiver or first part is moved the other pieces, to which the receiver is connected, must be constrained to move in the manner determined by their construction and by the mode of their connection. There are many hundreds of mechan ical movements employed in the construction of machines, but as it does not accord with the plan of this volume to present an exhaustive treatise in this department, we shall confine ourselves to the description of the few well-known forms illustrated on Plate 12o.
simplest mechanical element for giving movement and gaining advantage is the prying-lever, or " lever of the first kind," which is shown in Figure i. The point A is placed beneath the object to be raised, the heel resting upon a fixed piece (C) called the " fulcrum," and the power is applied to the longer projecting end B. With this kind of lever the weight to be raised moves in a direction opposite to that of the power. In Figure 2 is shown the lifting-lever, or " lever of the second kind," whose action is the same as that of the prying-lever, but the power moves in the same direction as the weight. In the " lever of the third kind" (fig. 3), the power also moves in the same direction as the weight, but it acts at a disadvantage, as the power is applied nearer the fulcrum, which is at the end of the lever and must be greater than the weight moved. In this method of application Nature antedates man's contriv ances, since his limbs are moved by muscular force applied in this way.