THE SCREW. 



95 



is considerable, and without it "it would not retain its 

 place, yet the slope of its inclined thread being so gi-ad- 

 ual, it possesses great power. This power is multiplied 

 to a still greater degree by the lever which is usually 

 employed to drive it, a {Fig. 76). If, 

 for example, a screw be ten inches in 

 circumference, and its threads half an 

 inch apart, it exerts a force twenty 

 times as great as the moving power. 

 If it be moved by a lever twenty 

 times as long as the diameter of the 

 screw, here is another increase of 

 twenty times in force. Multiplying 

 20 by 20 gives 400, the whole amount gained by this 

 combination, and by which a man applying one hund- 

 red pounds in force could exert a pressure equal to 

 twenty tons. About one tliird or 

 one fourth of this should, how- 

 ever, be deducted for friction. 



When the screw is combined 

 with the wheel and axle {Fig- 

 ure 77), it is capable of exert- 

 ing great power, which may be 

 readily calculated by multiply- 

 ^ , , ing the power of the screw and 



Screw, lever, and wheel " ^ 



combined. ifg Icvcr luto the power of the 



wheel and axle. 



