OF THE MECHANICAL POWERS. 71 



of the whole back is to the length of both the sides ; LECT. 

 supposing the wedge at the bottom of the cleft : or as v 

 the thickness of the whole back to the length of both 

 sides of the cleft, when the wood splits at any distance 

 before the wedge. For, when the wedge is driven quite 

 into the wood, and the wood splits at ever so small a 

 distance before its edge, the top of the wedge then be- 

 comes the acting part, because the wood does not touch 

 it any where else. And since the bottom of the cleft most 

 be considered as that part where the whole stickage or 

 resistance is accumulated, it is plain, from the nature of 

 the lever, that the farther the power acts from the resis- 

 tance, the greater is the advantage. 



Some writers have advanced that the power of the 

 wedge is to the resistance to be overcome, as the thick- 

 ness of the back of the wedge is to the length only of 

 one of its sides ; which seems very strange : for, if we 

 suppose A B to be a strong inflexible bar of wood or 

 iron fixed into the ground at C B, and D 

 and JE to be the two blocks of marble lying 

 on the ground on opposite sides of the bar ; 

 it is f-vident that the block Dmay be sepa- 

 rated from the bar to the distance d, equal 

 to a b, by driving the inclined plane or 

 half wedge a b o down between them ; and 

 the block E may be separated to an equal 

 distance on the other side, in like manner, 

 by the half wedge c d o. But the power 

 impelling each half wedge will be to the resistance 

 of the block against its side, as the thickness of 

 that half wedge is to its perpendicular height, because 

 the block will be driven off perpendicular to the side of 

 the bar A B. Therefore the power to drive both the 

 half wedges is to both the resistances, as both the half 

 backs is to the perpendicular height of each half wedge. 

 And if the bar be taken away, the blocks put close to- 



