MACHINE, an object by the intervention of which a motive power is made to act upon any body and overcome the force by which the latter resists the effort to change its state of rest or motion. A machine differs in no respect from a tool, an instrument, or an engine, and any one of these terms might be used indifferently for the same thing: the word tool is, however, generally applied to an object con taining in its construction some mechanical power, and which, when in use. is lucid in the hand of the operator.
The advantage which any machine affords for overcoming resistance, consists in the reaction by which it supporta a certain portion of the weight producing that resistance, so that the motive power has only to counteract the remainder. This may be immediately observed in those simple machines called the mechanical potters. For example : in the lever, the wheel and axle, and the pulley, the properties of which depend on the theory of parallel forces (when, consequently, of the resistance, the moving power, and the reaction of the machine, some one Is equal to the sum of the two others), any convenient portion of the resistance may be made to rest on the point of support, or the point of suspension. Again ; in the ineliued plane, the wedge, and the screw, the properties of which depend on the theory of forces concur ring in a point, the motive power, the resistance, and the reaction of the support, are represented by the three sides of a triangle ; and the ratio of the first to either of the others may be varied at pleasure by the construction of the machine.
The powers employed to give motion, through machinery, to any object, are produced by the muscular strength of men or animals; th actions of weights, springs, wind, steam, or fired gunpowder ; and these powers may generally be considered as pressures exerted during certain portions of time. Even that power which is produced by a sudden impulse, as when a rammer descending by its weight falls on the head of a pile, is only a pressure existing during an indefinitely short interval of time. The point in any machine to which the moving power is applied is called the impelled, and that against which the resistance acts is called the working point.
In the employment of any machine a certain portion of the power is expended in overcoming the inertia and friction of the materials, and that which remains is the only efficient force by which the useful effect is to be obtained. Thus, in pushing or drawing a body up an inclined plane, the effective motive power is less than that which is actually expended by as much as is necessary to overcome the inertia of the body and its friction on the plane; and these might be avoided if it were possible or convenient to raise the body vertically to an equal height by the descent of a body of greater weight, when both are connected together by a string passing over a pulley. The loss of power from inertia is doubled when a reciprocating motion exists in the same machine ; for a momentary state of rest takes place between every two contrary directions of the movement, and imme diately afterwards a .new inertia is to be overcome. The retarding forces above mentioned are evidently greater as the quantity of machinery in an engine is augmented ; and hence every machine should be as simple as possible consistently with the requisite relation between the moving power and the opposing resistance.
In the construction of machinery it is evident that all abrupt variations of velocity should be prevented, on account of the irregularity which they induce in the action. When, for example, one wheel drives another by means of the teeth on their circumference, the pressure of the teeth takes place wholly on one side of the latter, and the movement may be steady if the teeth are well formed ; but on a sudden diminution of the velocity of the driving-wheel, that which is driven, continuing for a time to move with its actual velocity, tends further to retard the movement of the other, and the pressure of the teeth against each other takes place on the opposite side. Thus a shaking motion is produced which diminishes the efficacy of the machine. The disadvantage attending such variations in the move ment of the machinery renders it advisable to gain the required effect by continued pressure, if possible, rather than by the employment of percussive forces.