Chap. 7.] Me We^t. 79 



raife, or to comprefs bodies ; and to put it in action 

 the blow or-ihock is commonly given with a hard 

 body, fuch as a (ledge or hammer, though fometimes 

 the prefiure of a weight is employed. The refjftance 

 which may be overcome by means of the wedge, of- 

 ten depends upon the tenacity of the parts, which is 

 difficult to eftimatei The percuffion which puts the 

 wedge into action is alfo difficult to judge of by the . 

 effects of preflure: on this account the theory of the 

 wedge is not fufceptible of great precificn. But ap- 

 proaches may be made to precifion, by fubftituting 

 powers, the abfolute force of which is known, as of 

 weights, and then obferving what proportion there 

 exifts between the power and the refiftance when a 

 wedge is introduced. 



Let us fuppofe two rollers m, n> (fig. 3.) the, one 

 m attached to the cord / m e, and' the other n to the 

 cord n i d, each bearing a weight of lolbs. _p and r> 

 and paffing over the pullies / and b ; and let us fup- 

 pofe alfo that the bafe a b of a wedge is equal to 

 the half of its height c h. It will then require a pref- % 

 {lire of 5 ibs. to keep this wedge in equiiibrio with the 

 fum of the two weights, which is equal to 20 Ibs, 

 and a little more than 5 Ibs. to fink the wedge its 

 whole depth c b t without making any allowance for 

 friction. It is evident by the conftruction, that while 

 the wedge is funk its whole depth c h, the two weights 

 ^>.and r will each rife one half of i /, which is equal 

 to a b, the bafe of the wedge. And as it is required, 

 in producing an equilibrium, that the power iliould 

 be to the refiftance in an inverfe ratio to tne velocity, 

 or to the fpace through which two bodies move in the 

 fame time, it is clear that in this cafe, the power 

 rnuft be to the refiftance as 'the half of the bafe is to 

 the height of the wedge. The (harper the wedge i?, 



therefore, 



