MECHANICAL Machines are instruments interposed between the moving power and the resistance, with a view of changing the direction of the force, or otherwise modifying it. Machines are of various degrees of complexity; but the simple parts, or elements of which they are all composed, are reducible to a very few. These elementary machines are called the MECHANICAL POWERS, and are usually reck oned as six in number, three beiug primary—viz., the lever, inclined plane, and pulley; and three secondary, or derived from the others—viz., the (derived front the lever), the wedge, and the screw (both derived from the inclined plane). To these some add toothed-wheels. What is special to each machine will be found under its name; a few observations applicable to all may appropriately be made here. 1. In treatino- of the theory of the lever and other mechanical powers, the question really examined° is, not what power is necessary to move a certain weight, hut what power is necessary to balance it; what force at P, for instance (see LEVER, fig. 1), will just keep W. suspended. This once done, it is obvious that the least additional force to P will suffice to begin motion. 2. In pure theoretical mechanics, 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 be polectly rigid, not bending or altering its form under any pressure. The motion of the machine is also supposed to be without friction. In practical mechanics, the weight of the machine, the yielding of its parts, and the resistance of friction, have to be taken into account. 3. When the effect of a machine is to make a force overcome a resistance greater than itself, it is said to give a meciatnical adcanteiye. A machite, however, never actually increases power—for that would be to create work or energy, n. thing now known to be as impossible as to create matter. What is gained in one way by a machine is always lost in another. One lb. at the long end of a lever will lift 10: lbs. at the short eud, if the arms are rightly proportioned; but to lift the 10 lbs. through 1 ft., it must descend 10 feet. The two weights, when thus in 'notion, have equal momenta; the moving mass multiplied into its velocity, is equal to the resisting mass multiplied into its velocity. When the lever seems to multiply force, it .only trates or accumulates the exertions of the force. The descending 1 lb. weight, in the case above supposed, may be coneeived as making 10 distinct exertions of its force, each • through a space of a foot; and all these are concentrated in the raising of the 10 lb weight through 1 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 pmssure; it is as often to gain velocity at the expense of force. See LEVER. In a spinning-factory, e.g., 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 mechanical advantage of a compound machine is theoretically equal to the product of the separate mechanical 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, electrical and magnetic attractions, steam, etc.; and the grand object in the construction of machines is, how, with a given amount of impelling power, to get the greatest amount of work of the kind required. Sec WORK, FOOT-POUND. This gives rise to a multitude of problems, some more or less general, others relating more especially to particular cases—problems, the investigation of which constitutes the science of applied mechanics. One of the questions of most general application is the following If the resistance to a machine were gradually reduced to zero, its velocity would be constantly accelerated until it attained a maximum, which would be when the point to which the impelling force is applied was moving at the. same rate RS the impelling forc,e itself (e.g., the piston-rod of a steam-engine) would move if unresisted. lf, on the other hand, the resistance were increased to a certain point,. the machine would come to a stand. Now the problem is, between these two extremes to find the rate at which the greatest effect or amount of work is got from the same 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. The moving force and the resistance should therefore be so adjusted as to produce this velodity.