MECHANICS is the science which treats of the nature of forces and of their action on.: bodies, either directly or by- the agency of machinery. The nature of force will be found treated of under,FoncE. The action of forces on bodies may be in the form of pressure or of impulse, and rnay or may not produce motion. When the forces are so balanced as to preserve the body affected by them in a state of equilibrium their actions are investigated in that branch of mechanics called STATICS (q.v.); when motion is produced, they are considered under the head of DYNAMICS (q.v.) or Kinetic& The equilibrium and motion of fluids (including liquids and gases) is treated in the subordinate branches of FIYDROSTATICS and fly-nnonyx_kmtcs; though the special tertns AEROSTATICS and AERODYNAMICS (for which the comprehensive term P.NEITMATICS iS often used) are some times employed to designate those portions of the science of mechanics in which the. action of gaseous bodies is treated of The science of mechanics owes very little to the ancient philosophers. They were. acquainted with the conditions of equilibrium on the lever—discovered by Archimedes- and had reduced the theory of all the mechanical powers, except the pulley and the inclined plane and its derivatives, to that df the lever, but this was nearly all. Archi medes, starting. from the principle of equilibrium on the lever, struck out the idea of a center of gravity for every body, and investigated the position of that point for the triangle, parabola, and paraboloid. Till the 16th c. the science remained stationary; Cardan, the marquis Ubaldi, and Stevinus--who was the first to give the correct theory of equilibrium on the inclined plane—then gave it a slight impetus, and the labors of Galileo, who introduced the expression of mechanical propositions in mathematical fortnulas, discovered the laws regulating the motion of falling bodies, and originated investiosations concerning the strength of materials, placed the science on a broad and substatitial basis. Torricelli, Descartes, Pascal, Fermat, Roberval, and Huygens, on the.
continent, and Wallis and Wren in England—the last three of whom simultaneously discovered the laws which regulate the collision of bodies—added each his quota to the. new science, as mechanics was then called. In 16S7 appeared Newton's Principia, in. which the complete experimental basis of the subject was first laid down in a satisfactory manner, and the mechanical prinSples, which had before been considered to act only at the surface of the earth, were shown to rule and direct the motions of the planets_ Contemporary with Newton were Leibnitz and the two elder Bernouillis, James and John, who, besides contributing greatly to the advancement of the science, applied to it the newly-invented differential calculus, which was found to be a weapon of immense power. From this time a constant succession of illustrious men have prosecuted the study- of theoretical mechanics, or of subjects connected with it. The chief names are Daniel Bernouilli, Euler, D'Alembert, Clairant, Lagrange, Laplace. Lagrange's Arecanique Analytique not only systematized the subject but enormously increased its power and the ranFe of its applications. The last great additions to the science are those made by Sir W. R. Hamilton (q.v.) under the name of the principle of vaiwing action. The developments which this has received from Jacobi, Boole, Cayley, Lion ville, Donkin, Bour, etc., form an extensive and difficult braneli of applied mathematics, chiefly orthe theory of simultaneous differential equations.