1 66 MAN'S WAY OF HELPING HIMSELF 



Because of these facts, engineers spend great sums in 

 grading down railroad beds, making them as nearly level as 

 possible. In mountainous regions, the topography of the 

 land prevents the elimination of all steep grades, but never- 

 theless the attempt is always made to follow the easiest grades. 



158. The Wedge. If an inclined 

 plane is pushed underneath or 

 within an object, it serves as a 

 wedge. Usually a wedge consists 

 of two inclined planes (Fig. 107). 



A chisel and an ax are illustra- 

 tions of wedges. Perhaps the most 

 universal form of a wedge is our 

 common pin. Can you explain how 



FlG. 107. Bv means of a wedge, . 



the stump is split. this IS a Wedge ? 



159. The Screw. Another valua- 

 ble and indispensable form of the inclined plane is the screw. 

 This consists of a metal rod around which passes a ridge, and 

 Figure 108 shows clearly that a screw is simply a 



rod around which (in effect) an inclined plane has 

 been wrapped. 



The ridge encircling the screw is called the 

 thread, and the distance between two successive 

 threads is called the pitch. It is easy to see that 

 the closer the threads and the smaller the pitch, 



FIG. 108. 



the greater the advantage of the screw, and hence A screw as 

 the less force needed in overcoming resistance. a SIm P Ie 



machine. 



A corkscrew is a familiar illustration of the screw. 



160. Pulleys. The pulley, another of the machines, is 

 merely a grooved wheel around which a cord passes. It is 

 sometimes more convenient to move a load in one direction 

 rather than in another, and the pulley in its simplest form 

 enables us to do this. In order to raise a flag to the top of 



