a. OAONPO FA Se A

therefore, and dimenfions mutt be deter- mined by the ftruture of the machine. it moves, and the direétion of the force that acts upon it, which are the only circum- ftances that can affect it.

Ifa wedge were driven into any body. whofe preffure and friction are equally di- {perfed overthe fides of the wedge, common fenfe and experience dictate, that, to move it ftraight forward, the force muft be ap- plied, as near as poflible, to the middle of sts back. But, if the preflure and fri€tion are not equally difperfed, or take place more towards one fide of it than the other; then the force mutt be applied on the back, as much nearer that fide as the preflure and friGion on it are greater. All which in theory means no more than this, that, to move the wedge ftraight forward, the force mutt be applied fo as to move the cen-

ter of refiftence* in that direction. In

* The center of gravity, is the point at which allthe weight of a body may be fuppofed accu- mulated, on its being fufpended. The center of refiftence,