MECHANICAL POWEBS. 242 MECHANICAL UNITS. but the tliiid and fourth are true of machines only and serve to distinguish machines from structures. A structure modifies and transmits force onl}-, and does not permit rchitivo motion of its members; a machine iiioditics 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
- nembers. A mechanism does work incidentally
such as the overcoming of its own frictional resistance; its primary function is to modify and transmit motion; a meclianisin or combination of mechanisms which receive energy and transmit and modify it for the performance of useful work constitute a machine. ^lacliines are of various degrees of complexity, but the simple parts or elements of which they are composed 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 necessary 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 suflice to be- gin 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 pcr- f'-clly 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 weiglit of the machine, the yieliling of its parts, and the resistance of fric- tion have to be taken into account. (3) When the elTect of a machine is to make a force over- come a resistance greater than itself, it is said to give a mechanical advantage. A machine, however, never actually increases power — for that would be to create work or energy, a thing as impossible as to create matter. What is gained in one way by a machine is always lost in another. One 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 i)onnds through I foot, it nuist 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 case above supposed, may be conceived as making ten ilistini't exTtions of its force, each through a space of a foot; and all these are concentrated in the raising of the ten-pound weight through one font. 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 exjiense of force. (See Leveh. ) 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 macliine is theo- retically 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 comjiosing them, the flexure of parts sulijected to strains, and the friction, which increases rapidly with the complexity of the parts; and these considi'rations 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, with a given amount of impelling power, to get the greatest amount of work of the kind required. (See Work; Koot-i-oi xd. ) This gives rise to ti multitmle of |)rubleiiis. some more or less gen- eral, others relating more especially to particu- lar ca.ses — problems the investigation of which constitutes the science of applied mechanics. One of the questions of most general ajiplica- tion is the following: If the resistance to a machine were gradually retluced to zero, its velocity would be constantly accelerated tuilil it attained :i maximum, wlilch would be 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) would move if unresisted. If, on the other hand, the resistance were increased to a certain point, the machine w'ould come to a stand. Xow the problem is, between these two extremes to find the rate at which the greatest efTect or amount of work is got from the same amount of driving power. The investigation would be out of place Iierc, but the result is that the greatest elTect is pnxluced when the velocit,v of the point of application is one-third of the maximum velocity above spoken of. The moving force and the re- sistance should therefore be so adjusted as to prcKhicc this v<'Ioc-ilv. Sec Mechanics. MECHANICAL TISSUE. The suiiporting tissue (sterconie) of the ]dant, including not onl,v the vascular system, but also the cortical sclerenchyma and colhnchyma. Cortical me- chanical tissue is ])articularly prominent in the hvpodcrma nf stems and leaves. 'MECHANICAL UNITS. Various units or standariN u-ril in dill'erent countries and under dill'crent conditions for the expression of me- chanical quantities. One system, the C. G. S. system (q.v.). is based upon the centimeter, the gram, ami the mean solar second, . other uses the foot, the pound, and the mean solar second. The yard and the ])oun<l are legally defined as follows: "The straight line or distance be- tween the centres of the transverse lines in the two gohl plugs in the l)ronze bar depositeil in the ollice of the Excherpier [London] shall be the gentiine standard yard at 62° F. ; the pound is the mass of a certain piece of platinum marked
