MEASUREMENT OF WORK 



171 



FlG. 114. By means of a belt, motion 

 can be transferred from place to place. 



An extremely important form of wheel and 'axle is that in 

 which the two wheels are connected by belts as in Figure 1 14. 

 Rotation of W 7 induces rotation 

 of w, and a small force at W is 

 able to overcome a large force 

 at w. An advantage of the 

 belt connection is that power 

 at one place can be transmitted 

 over a considerable distance 

 and utilized in another place. 



166. Compound Machines. Out of the few simple machines 

 mentioned in the preceding Sections has developed the complex 

 machinery of to-day. By a combination of screw and lever, for 

 example, we obtain the advantage due to each device, and some 

 compound machines have been made which combine all the va- 

 rious kinds of simple machines, and in this way multiply their me- 

 chanical advantage many fold. 

 A relatively simple com- 

 plex machine called the crane 

 (Fig. 116) maybe seen almost 

 any day on the street, or 

 wherever heavy weights are 

 being lifted. It is clear that a 

 force applied to turn wheel I 

 causes a slower rotation of 

 wheel 3, and a still slower 

 rotation of wheel 4, but as 4 

 rotates it winds up a chain and 



FIG. 



115. A simple derrick 

 weights. 



raising 



167. Measurement of Work. 



slowly raises Q. A very com- 

 plex machine is that seen in 

 Figure 117. 

 In Section 150, -we learned 



that the amount of work done depends upon the force 



