REST

, in Physics, the continuance of a body in the same place; or its continual application or contiguity to the same parts of the ambient and contiguous bodies.—See Space.

Rest is either absolute or relative, as place is.

Some define Rest to be the state of a thing without motion; and hence again Rest becomes either absolute or relative, as motion is.

Newton defines true or absolute Rest to be the continuance of a body in the same part of absolute and immoveable space; and relative Rest to be the continuance of a body in the same part of relative space.

Thus, in a ship under sail, relative Rest is the continuance of a body in the same part of the ship. But true or absolute Rest is its continuance in the same part of universal space in which the ship itself is contained.

Hence, if the earth be really and absolutely at Rest, the body relatively at Rest in the ship will really and absolutely move, and that with the same velocity as the ship itself. But if the earth do likewise move, there will then arise a real and absolute motion of the body at Rest; partly from the real motion of the earth in absolute space, and partly from the relative motion of the ship on the sea. Lastly, if the body be likewise relatively moved in the ship, its real motion will arise partly from the real motion of the earth in immoveable space, and partly from the relative motion of the ship on the sea, and of the body in the ship.

It is an axiom in philosophy, that matter is indifferent as to Rest or motion. Hence Newton lays it down, a<*> a law of nature, that every body perseveres in its state, either of Rest or uniform motion, except so far as it is disturbed by external causes.

The Cartesians assert, that firmness, hardness, or solidity of bodies, consists in this, that their parts are at Rest with regard to each other; and this Rest they establish as the great nexus, or principle of cohesion, by which the parts are connected together. On the other hand, they make fluidity to consist in a perpetual motion of the parts, &c. But the Newtonian philosophy furnishes us with much better solutions.

Maupertuis asserts, that when bodies are in equilibrio, and any small motion is impressed on them, the quantity of action resulting will be the least possible. This he calls the law of Rest; and from this law he deduces the fundamental proposition of statics. See Berlin Mem. tom. 2, pa. 294. And from the same principle too he deduces the laws of percussion.