MECHANICAL POWERS — MACHINES. Technically described a machine is a combination of resistant bodies for modifying energy and doing work. the members of which are so ar ranged that, in operation, the motion of any member involves definite, relative, constrained motion of the others. A brief analysis of this definition will help to make it clear. First, a machine must consist of a combination of bodies; thus a lever must have its fulcrum, a screw its nut, a wheel and axle its bearings, and so on with other examples; the simplest machine must have at least two members between which relative motion is possible. Second, the members of a ma chine must lie resistant in order to transmit force: they generally are rigid. hut not necessarily so, since flexible belts. chains, or springs may he employed to transmit force under the par ticular action to which they are adapted. Third, a machine is used to modify energy and perform work. This proposition is obvious. The con ception of a machine involves the conception of some source of energy and a train of mechanism suitably arranged to receive, modify, and apply the energy derived from this source to the de sired end. A machine, then, consists of (1 parts receiving the energy: (2) parts transmit ting and modifying the energy; and (3) parts performing the required work. Finally, (4) the relative motions of the members of a machine are constrained or restricted to certain definite, pre determined paths in which they must move, if they move at all, relatively. The first tw•n propositions of the definition are equally true of structures (such as a bridge) as of machines, but the third and fourth are true of machines only and serve to distinguish machines from structures. A structure modifies and transmits force only, and does not permit relative motion of its members; a machine modifies and trans mits force and motion, that is, energy, and per mits relative constrained motion of its members.
The distinction between a machine and a mechanism remains to be explained. A mechan ism is a combination of resistant bodies for trans Mitting and modifying motion (not motion and force or energy as in machines) so arranged that, in operation, the motion of any member involves definite, relative, constrained motion of the other members. A mechanism does work incidentally such as the overcoming of its own frictional resistance; its primary function is to modify and transmit motion; a mechanism or combination of mechanisms which receive energy and transmit and modify it for the performance of useful work constitute a machine.
Machines are of various degrees of complexity, but the simple parts or elements of which they are eomposed are reducible to a very few. These elementary machines are called the mechanical powers, and are commonly reckoned as the lever, the inclined plane, the jointed link, or toggle joint, and the hydraulic press. All ma chines and all locomotive movements of ani mals resolve themselves into the action of one or a combination of these simple mechanical powers or machine elements. A few observations applicable to all may appropriately be made here. (1) In treating of the theory of the lever and other mechanical powers, the question really examined is, not what power is nevessary to move a certain weight, but what power is neces sary to balance it. This once done, it is obvious that the least additional force will suffice to be gin motion. (2) In pure theoretical meehanies, it is assumed that the machines are without weight. A lever, for instance, is supposed to be a mere rigid line; it is also supposed to he per frelly rigid, not bending or altering its form under any pressure. The motion of the machine is also supposed to be without friction. in prac tical mechanics, the weight of the machine, the of its !tarts, and the resistance of fric tion have to be taken into account. (3) When
the effect of a machine is to make a force over come a resistance greater than itself, it is said to give a mechunieol advantage. A machine, however, never actually increases power—for that would be to create work or energy, a thing as impossible :is to create matter. What is gainetl in one way by a machine is always lost in another. (Inc pound of weight at the long end of a lever will lift 10 pounds at the short end, if the arms are rightly proportioned; but to lift 10 pounds through 1 foot, it utast descend 10 feet. The two weights, when thus in motion, have equal momenta ; the moving mass multi plied into its velocity is equal to the resisting mass multiplied into its velocity. When the lever seems to multiply force, it only concen trates or accumulates the exertions of the force. The descending one-pound weight. in the ease above supposed, may be v01101.111.4 as making ten distinct ev.rtions of its force, each through a space of a fold; and all these are concentrated in the of the ten•pound weight through one foot. The principle thus illustrated in the case of the lever holds good of all the other mechanical powers. (4) The object of a machine is not always to increase force or pressure; it is as often to gain velocity at the expense of force. (See LEvEtt.) In a factory, for example, the object of the train of machinery is to distribute the slowly working force of a powerful water wheel or other prime mover, among a multitude of terminal parts moving rapidly, but having little resistance to overcome. (5) The mechani cal advantage of a compound machine is theo retically equal to the product of the separate meelianieal advantages of the simple machines composing it; but in applying, machines to do work, allowance must be made for the inertia of the materials composing them, the flexure of parts subjected to strains, and the friction, which increases rapidly with the complexity of the parts; and these considerations make it desirable that a machine should consist of as few parts as are consistent with the work it has to do, (6) The forces or 'moving powers' by which machines are driven are the muscular strength of men and animals, wind, water, electrieal and magnetic attractions, steam, etc.; and the grand object in the construetion of maehines is, with a given amount of impelling power, to get the greatest aniount of work of the kind required. (See Worn:: FooT-rot'xii.) This gives rise to a multitude of problems, some more or less gen eral, others relating more especially to particu lar eases—problems the investigation of whieh constitutes the science of applied mechanics. One of the questions of most general applica tion is the following: If the resistance to a machine were gr.olually reduced to zero. its Velocity W011111 be constantly accelerated until it attained a ina‘inuain, which would he when the point to which the impelling force is applied was moving at the same rate as the impelling force itself (e.g. the piston-rod of a steam-engine) NV011111 1110Ve if unresisted. Tf, on the other hand, the resistance were increased to a certain point, the machine would collie to a stand. Now the problem is, between these two extremes to timid the rate at which the greatest effect or amount of work is got from the slime amount of driving power. The investigation would be out of place here, but, the result is that the greatest effect is produced when the velocity of the point of application is one-third of the maximum velocity above spoken of. 'fine moving force and the re sistanve should therefore be so adjusted as to produce this velovity. See 11::cifAx1( 'S.