MACHINES 543 



quickly. The sewing machine enables a woman to operate 

 the needle by moving her foot while both hands are free to 

 handle the work; moreover, the needle makes several stitches 

 while her foot is making a single motion (Art. 619, Ex. 101). The 

 plow, the cultivator, the mowing machine, or the binder en- 

 ables the farmer to utilize the efforts of horses. Steam engines, 

 gas engines, electric dynamos and motors enable man to utilize 

 the energy in fuel at a small fraction of the cost of hiring the 

 same amount of work done by manual labor or even by horses 

 (Fig. 339). Moreover, one man often operates such a machine 

 while it does the work which would otherwise have to be done 

 by hundreds of men. Waterwheels enable men to utilize the 

 energy in running water energy which would otherwise go to 

 waste. When this energy has been transformed into elec- 

 tricity it can be transmitted on wires many miles and then be 

 used to light our homes, run our trains and street cars, and 

 do a large portion of the work which has been done by men 

 in days past (see Arts. 668 and 673). 



641. Mechanical Advantage of a Machine. Man uses a 

 machine only when he gains some advantage by so doing. This 

 advantage gained by using a machine is called the MECHANICAL 

 ADVANTAGE of the machine. Machines may offer mechanical 

 advantage of several different kinds: 



1. A machine has MECHANICAL ADVANTAGE OF FORCE when, 

 by using it, a greater force is exerted than could otherwise be 

 exerted. Examples: By using a pinchbar a crowbar of a 

 certain shape a man is able to move a heavily loaded freight 

 car which he could not move without it. By using a common 

 claw hammer, a man can draw a nail which otherwise he could 

 not pull. Give as many other examples as possible. 



2. A machine has MECHANICAL ADVANTAGE OF SPEED when 

 it is used to increase the speed with which the force acts. 

 Examples: A fly swatter, a sewing machine, an egg beater, 

 an old-fashioned spinning wheel. Name as many other 

 examples as you can. 



3. A machine may have MECHANICAL ADVANTAGE OF DIREC- 



