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ON THE PLOUGH. 35

In the fame manner, if a wedge were moved through fand, or fuch a body as earth, on:one of its edges, and had more preffure or friction on its lower‘than its higher parts, to make it hold a horizontal direction, the force behoved to be applied on its back, as much lower than the middle, as the refiftence on that fide is greater. Now, if inftead of the force applied to its back, it were pulled forward by an equal force applied to its face, this force muft act in a horizontal line beginning from its cen- ter of refiftence. Hence it appears, that, were the plough a perfect half-wedge, the

_ only direétion of the force that could move

it ftraight forward, would be a_ horizontal one, pafling as much below the furface of the foil as this center is fituated below.it; and that a direétion fo much above this, as

that

refiftence, isa point at which is accumulated all oppofition to its motion forward. This oppofition arifes from the inertia of the body itfelf, and the refiftence and friction of other bodies. The center of refiftence will always approach that fide on which the obftru€tions are greater.