203 



LECTURE XVIII. 



ON RAISING AND REMOVING WEIGHTS, 



The methodical arrangement of our subject leads us, after having consi- 

 dered the modifications of force, to those machines which are intended for 

 counteracting 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 of 

 the most common employments of mechanical powers to raise a weight from a 

 lower to a higher situation. This operation is also intimately connected with 

 the modes of overcoming the corpuscular force of friction"^r adhesion, which 

 constitutes the principal difficulty in removing bodies horizontally from place 

 to 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 of 

 avoiding friction in general, constitute the first part of the subject of the 

 counteraction of forces, and the remaining part relates to the machinery in- 

 tended'ifor overcoming the other corpuscular powers of bodies, by such opera- 

 tions as are calculated to change their external forms. 



Machines for raising Aveights, which involve only the mechanics of solid 

 bodies, are principally levers, capstans, wheels, puUies, inclined planes, screws, 

 and their various combinations, in the form of cranes. 



A lever is a very simple instrument, but of most extensive utility in raising- 

 weights to a small height. We may recollect that levers are distinguished 

 into 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 

 other being in the one case in the same direction, in the other, in opposite 

 directions. Thus, when a man lifts a stone by means of a lever of the first 

 kind, resting on a fulcrum between himself and the stone, he presses down 



