306 



HOW MACHINES LIGHTEN LABOR 



which the two wheels are connected by belts, as in Figure 176. 

 Rotation of W 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. 



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 various 

 kinds of simple machines, and 

 in this way multiply their ad' 

 vantage manyfold. 



A relatively simple com- 

 pound machine called the crane 



FIG. 177. A simple derrick for raising (fig. 179) may be seen almost 

 weights. & 



any day on the street, or wher- 

 ever heavy weights are being lifted. Two of the most common 

 illustrations of compound machines are the sewing machine and 

 the typewriter. Can you pick out the simple machines in each ? 

 Measurement of work. We have learned that the amount 

 of work done depends upon the force exerted, and the distance 

 covered. A man who raises 5 pounds a height of 5 feet does 

 far more work than a man who raises 5 ounces a height of 5 

 inches, but the product of force by distance is 25 in each case. 

 There is difficulty because we have not selected an arbitrary 

 unit of work. The unit of work used in practical affairs is the 

 foot pound. It is defined as the work done when a force of I 

 pound acts through a distance of I foot. A man who moves 8 



