292 



HOW MACHINES LIGHTEN LABOR 



The amount of work done depends upon the force used and 

 the distance covered, and hence we can say that 



or 



Work = force multiplied by distance, 



W=fxd. 



Machines. A glance into our machine shops, our factories, 

 and even our homes shows how widespread is the use of com- 

 plex machinery. But all machines, 

 however complicated in appearance, 

 are in reality but modifications and 

 combinations of one or more of four 

 simple machines devised long ago 

 by our remote ancestors. These 

 simple devices are known to-day, 

 as (i) the lever, represented by a 

 crowbar, a pitchfork; (2) the in- 

 clined plane, represented by the 

 plank upon which barrels are rolled 

 into a wagon; (3) the pulley, rep- 

 resented by almost any contrivance 

 for the raising of furniture to upper 

 stories ; (4) the wheel and axle, rep- 

 resented by cogwheels and coffee 

 grinders. 



Suppose a i6oo-pound bowlder 

 which is embedded in the ground 

 is needed for the tower of a build- 

 ing. The problem of the builder is 

 to get the heavy bowlder out of the ground, to load it on a 

 wagon for transportation, and, finally, to raise it to the tower. 

 Obviously, he cannot do this alone; the greatest amount of 

 force of which he is capable would not suffice to accomplish 

 any one of these tasks. How, then, does he help himself and 



FIG. 155. Primitive method of 

 grinding corn. 





