70 MECHANICAL POWERS. 



ments. The ancients occasionally drew up their armies 

 in the shape of a wedge, in order to break the line of 

 their opponents. 



The Screw is a kind of inclined plane wound round a 

 cylinder, the power of which is increased by a handle or 

 lever. The uses to which the screw is applied are various; 

 bodies are pressed together by it, as with the common 

 table cloth and napkin press, the bookbinder's press, &c. 

 The screw is also sometimes used to raise the parts of a 

 building that may have settled or sunk, through the 

 foundation giving way ; and if the material be sufficiently 

 strong, one man with a good leverage may raise an im- 

 mense weight. The power gained by the screw is as the 

 circumference made by the moving power is to the dis- 

 tance between the threads of the screw : thus, if the 

 distance between the threads of the screw be half an 

 inch, and the lever used be three feet, the circumference 

 made by the moving power will be eighteen feet, or '216 

 inches. The advantage obtained will then be as 216 are 

 to one-half, or as 432 to one, so that a man who can push 

 with the force of a hundred weight may produce a power 

 (setting friction aside) nearly equal to 22 tons. 



In the screw and the wedge there is a very great fric- 

 tion to overcome, independent of the weight, sometimes 

 as much as nearly amounts to the weight raised ; but 

 then the friction is of use to retain the weight and the 

 machine in their places after the power is removed. If 

 machines could be made without friction, the least de- 

 gree of power beyond that which balances the weight, 

 would be sufficient, to raise it. 



In the lever, the friction is very little, as also in the 

 wheel and axle; in pulleys it is very considerable, and in 

 the inclined plane, wedge, and screw, it is very great. In 

 machines in general one-fourth, and sometimes one-third, 

 is allowed for friction. Mr. Wyatt considers one-fourth 

 lost in a steam-engine by friction. In pulleys, friction 

 may be reduced one-half by grease. The least friction 

 is produced when polished iron moves on brass. 



The mechanical arts are of the greatest importance to 

 mankind, and the extent to which machinery has been 

 carried is truly wonderful. It is by a skilful combination 



