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LECTURE XVIII

ON RAISING AND REMOVING WEIGHTS.

TlIE methodical arrangement of our subject leads us, after having consi-dered the modifications of force, to those machines which are intended forcounteracting it, or for producing motion in opposition to an existing force.The simplest of the forces to be counteracted, is gravitation, and it is one ofthe most common employments of mechanical powers to raise a weight from alower to a higher situation. This operation is also intimately connected withthe modes of overcoming the corpuscular force of friction or adhesion, whichconstitutes the principal difficulty in removing bodies horizontally from placeto place; for if we had only to produce motion in an unresisting mass of mat-ter, a loaded waggon might in time be drawn along by a silk worm’s thread.The raising and removing of weights, therefore, together with the modes ofVoiding friction in general, constitute the first part of the subject of thecounteraction of forces, and the remaining part relates to the machinery in-tended for overcoming the other corpuscular powers of bodies, by such opera-hons as are calculated to change their external forms.

Machines for raising weights, which involve only the mechanics of solidIndies, are principally levers, capstans, wheels, pullies, inclined planes, screws,'ffid their various combinations, in the form of cranes.

A lever is a very simple instrument, but of most extensive utility in raising-heights to a small height. We may recollect that levers are distinguishedln to two principal kinds, accordingly as the power and weight are on differ-ent sides, or on the same side of the fulcrum; the forces counteracting each^her being in the one case in the same direction, in the other, in oppositeSections. Thus, when a man lifts a stone by means of a lever of the first^ted, resting on a fulcrum between himself and the stone, he presses down

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