COMPOUND MACHINES. 



Ill 



209. Law of the Screw. The second general 

 law of machines ( 167, [2]) may be adapted to our present 

 purpose as follows : With the screw, a given power wi% 

 support a weight as many times as great as itself as 

 the circumference described l)ij the power is times as 

 great as the distance between two adjoining turns 

 of the thread. 



210. The Endless Screw. An endless screw 

 is one whose thread acts on the teeth of a wheel. 

 The screw has a rotary but no 



lengthwise motion. As the han- 

 dle is turned, the thread catches 

 the teeth and turns the wheel. 

 The wheel moves one tooth for 

 every turn of the handle. Suc- 

 cessive teeth are caught as others 

 pass out of reach. A continuous 

 motion is thus produced ; hence 

 the name " endless screw." The 

 figure will aid in the application of the second general law 

 of machines to determine the ratio between the weight and 

 the power. 



211. Compound Machines. We have now con- 

 sidered each of the six traditional simple machines. One 

 of these may be made to act upon another of the same 

 kind, as in the case of the compound lever or wheel-work ; 

 or upon another of a different kind, as in the case of the 

 endless screw. When any two or more of these machines 

 are combined, the effective force may be found by comput- 

 ing the effect of each separately and then confounding 

 them ; or by finding the weight that the given power will 



FIG. 62. 



