62 



GENERAL SCIENCE 



are put on low-wheeled carriages and moved up the 



plane. 



The Screw. Cut a piece of paper in the form of a 



right-angled triangle (Figure 58) ; the side C is an in- 

 clined plane. Wind the paper 

 around a pencil, and it has the 

 appearance of a screw. Can you 

 follow the inclined plane from 

 the bottom to the top of the 

 pencil? The elevation made in 

 one turn is called a thread. 



A stairway is a common form 

 of inclined plane. Stairways are 

 often arranged in towers and 

 lighthouses in the form of a 



FIG. 58. Showing the Principle 

 Involved in the Screw. 



spiral, the inclined plane turning round and round the 

 inside of the tower. The reason the screw is so powerful 

 is that the power moves through such a long distance 

 while the screw moves a very small distance. In the 

 jackscrew in Figure 59, suppose 

 the handle to be 4 feet long and 

 the pitch of the screw one half 

 an inch. Then as the end of the 

 handle moves around the circle 

 25 feet (4 X 2 X 3.1416) in cir- 

 cumference, the screw is moved 

 up one half an inch. A force 

 of one pound exerted at the end FlG - 59 Liftin Jack ' 

 of the handle will lift 600 pounds at W, since one half an 

 inch is contained 600 times in 25 feet. 



The wedge is a machine used for splitting and forcing 

 materials apart where great power is required. Figure 

 60 illustrates the use of a wedge. If the wedge is 12 inches 



