THE SCREW 181 



together. It is made of hard wood or of iron, and is used to split logs 

 and stones, or sometimes to raise a heavy weight. If the length of the 

 wedge is 5 times its thickness, we drive it into the log 5 inches in 

 order to force the wood apart 1 inch. The axe is a wedge; so are the 

 pin and the needle (cf. 196). 



205. The Screw. If we were going to the top of a 

 lighthouse 100 feet high, we might climb a vertical ladder 

 or rope, or we might go up gradually, on a long inclined 

 plane, rising, say, 1 foot in every 10. In the ascent by the 

 inclined plane we would walk 1000 feet in 

 order to rise 100 feet. The most compact 

 way of arranging an inclined plane is in a 

 spiral. This is done in the spiral staircases 

 of many lighthouses and towers. 



Now, the screw (Fig. 183) is really a 

 long inclined plane with the ascent FIG. 

 arranged in a spiral instead of in a straight in ^8 nu 



1- A 11 < i i great weights, such 



line. A small force can produce a large as wagons or houses, 



ty , .., ., ,. for short distances. 



enect with the screw; for the power must 

 go a long distance in order to make the screw itself ad- 

 vance a very short distance. 



The distance between threads is called the " pitch " of the screw. 

 Suppose that a screw having 12 threads to the inch is driven by a 

 screw driver having a handle 2 inches in diameter. 

 The handle has a circumference of 2X37?, or 6 2 /? 

 inches. The handle is thus turned through 6 2 /7 

 inches while the screw advances Yi 2 of an inch. 

 Hence the power moves about 75 times (6 2 /7 divided 

 by Vi 2 ) as far as the resistance or weight; a force of 

 3 pounds applied on the handle will exert a force of 

 FIG. 184. about 225 pounds on the threads of the screw. 



The Letter Copying r i i /. i , , . 



Press: a Screw. The screw is used not only for holding pieces of 



