MECHANICS is the science in which are investigated the actions of bodies on one another, either directly or by means of machinery.
These actions may be simple pressures without motion, as when one body being supported in any manner, another is placed upon it, either vertically or in some oblique position ; or they may be such as are accompanied by motion, and these may arise either from the mutual attractions which all bodice in nature exercise upon each other, or from the collisions of bodies in motion with others which may be previously in motion or at rest.
The term is particularly applied to the mutual actions of solid bodies: the actions of fluids on solids form, in part, the subjects of HYDROSTATICS and FlrenonreaM cs; but these, as well as pneumatics, are now frequently included under the general name mechanics. When bodies are at rest, and the actions aro such as to maintain them in that state, the laws of the actions constitute that branch which is called Siena ; but when motion is concerned, the laws and phenomena constitute what is called Dr:seems.
In all the branches of general mechanics the investigations are founded on experiment and are oonducted by geometrical or alge braical processes: hence the science forms one of the departments of experimental philosophy, and also of mixed mathematics. This last denomination is applied to mechanics because in the latter are involved several qualities of bodies which do not enter into the researches of pure science, such as mass or quantity of material, inertia, hardness, elasticity, time, space, and power or force. The term is used to express the cause of the actions of bodies on one another; but we have no other conception of it than that it is productive of motion or of a tendency to motion, or that it arrests an actual motion or renders a tendency to motion ineffectual. When opposing forces act on a body so as to destroy each other's effects and keep the body unmoved, that body is maid to be in a state of equilibrium ; and this state is distin guished from that of mere rest, since the latter implies the absence of any cause by which the body mipht change its place.
The invention of simple machines for moving masses of any material which it might be beyond the unaided power of man to transport to a distance, must have taken place in the earliest ages of the world. Hun= ingenuity would readily suggest the application of a rod or bar of wood or Iron, for the purpose of lifting a heavy body from the ground; and very little expenenco would suffice to show that rolling or pushing a block of stone up sloping ground was a much easier ope ration than that of raising it vertically by the strength of men's arms.
Thus may have arisen the employment of the lerer and inclined plane; and from thew, subsequently, the wheel and axle, the pulley, the wedge, and the screw, were derived. The simple means here indicated would be sufficient, with the aid of manual labour, to build up the most massive cyclopean edifice; and even the vast materials which form the roofs of the Egyptian temples may have been raised to their places by means of inclined planes, formed of earth for the purpose, on the exterior of the walls, and afterwards removed.
The steps by which the art of constructing machines advanced have not been distinctly recorded ; and the work of Vitnivitia on archi tecture is almost the only source whence can be obtained an account of such as were in use in and before the time of that engineer (the age of Augustus or Vespasian). From the descriptions there given, it appears that among the mechanical powers then in use were the lever, the windlass, and the assemblage of pulleys. Vitruvius also mentions some ingenious contrivances for transporting heavy blocks of atone from the quarries, and a the invention, of which be ascribes to Ctesiblus, for supplying the publie fountains. He describers a complex machinery, consisting of whale driving each other by cogs or teeth, which was applied to carriages or ships for the purpose of measuring the distances travelled or sailed; and he enters fully into the construction of engines for throwing darts or masses of stone. Tho muscular strength of men was then employed as a moving power in turning mills : Oath impelled by ricer currents acting on float-boards (pewter) gave motion to machinery for grinding corn ; and wheels turned by men walking on them were used for raising water by buckets or otherwise. Vitruvius generally mentions the names given by the Greeks to the machinery; and it might, without great risk of error, be presumed that much of that which he describes was in use among the latter people at, or even before, the time when the Parthenon was raised. There are no distinct intimations of the existence of windmills till the 12th century. The expansive force of steam can only be said to have become a moving power at the end of the 17th century, and. then it was employed merely to raise water. Its general application to machinery must be dated from the year 176S.