i 82

Mechanics

fist of one, two, or more of the SimplePowers combined together; that in most

of

overcome the Friction would be to double the Power G,or H at once.

8. But though we may at a Medium allow about athird Part of the Weight with which any simple Ma-chine is charged from the Friction arising from thence,yet this is very precarious and seldom the Case ; for if

PI. X. A B C D be a Piece of Brass of six Ounces, and EFGH

Fig. i. be also a Plate of Brass, and both the Surfaces well

ground and polish’d, the Weight P of near two Ounceswill be requir’d to draw along the Body AC alone;but if AC be loaded with 6, 8, or io Ib. then a sixthPart of the Weight will be sufficient to draw it alongthe Plane. If the Plane be covered with a linen orwoollen Cloth, then a third, or half Part, and some-,times more, will be requisite to draw it along on thePlane.

y. Yet notwithstanding the Difficulty and Uncer-tainty attending the Estimation of the Quantity ofFriction, ’tis still a most useful and necessary Enquiryhow, and by what Means the Friction of any Machinemay be abated or diminished. In order to this we mustconsider it mechanically, that is, we must consider Fric-tion as a Force acting against a Power applied to overcomeit. Thus suppose AB an upright Stem or Shaft turn-ing freely in the Socket B fix’d in the Table or PlaneIK LM j and AC, DE two Arms fix’d in the saidShaft, the latter of which DE has three Pins goinginto a Socket in the Middle of a heavy Weight at F,G, or H, in such a Manner, that when a Power ap-plied at C moves the Lever AC, it causes the LeverIDE to protrude or thrust along the Weight at F, G,or H, in a circular Manner upon the Table.

io. Now since we suppose the Weight all the whileit is in Motion is freely and wholly supported by thePlane, it follows that all the Resistance it can give tothe Power applied at C, is only what arises from itsFriction on the Plane. What this Friction is, will be

3

found