MECHANIC POWERS. 45 



tween the power impelling the wedge downward 

 and the resistance of the wood acting against the 

 two sides of the wedge, when the power is to the 

 resistance, as half the thickness of the wedge at 

 its back is to the length of either of its sides ; be- 

 cause the resistance then acts perpendicular to the 

 sides of the wedge. But when the resistance on 

 each side acts parallel to the back, the power that 

 balances the resistances on both sides will be, as 

 the length of the whole back of the wedge is to 

 double its perpendicular height. 



When the wood cleaves at any distance before 

 the wedge (as it generally does) the power impell- 

 ing the wedge will not be to the resistance of the 

 wood as the length on the back of the wedge is to 

 the length of both its sides, but as half the length 

 of the back is to the length of either side of the 

 cleft estimated from the top or acting part of the 

 wedge. For, if we suppose the wedge to be 

 lengthened down from the top C . E to the bottom, 

 of the cleft at D, the same proportion will hold^ 

 namely, that the power will be to the resistance, as 

 half the length of the back of the wedge is to the 

 length of either of its sides: or, which amounts to 

 the same thing, as the whole length of the back is 

 to the length of both the sides. 



The wedge is a mechanic power of great effect, 

 since not only wood, but even rocks, can be split 

 by it - f which it would be impossible to accomplish 

 by the lever, wheel and axle, or pulley; for the 

 force of the blow or stroke shakes the cohering 

 parts, and thereby makes them separate more 

 easily. 



THE SCREW. 



The screw can scarcely be called a simple ma- 

 chine, because it is never used without the appli- 



