52 MOVING POWERS IN MACHINERY. 



Sometimes the same wheel acts both as a leader 

 and a follower; as in Fig. 11, where B is moved by 

 A, and consequently is a leader; while, as it drives 

 C, it is also a follower. Therefore, as to multiply 

 both the divisors and dividend by the same number 

 does not alter the quotient, so in mechanical cal- 

 culations, every wheel that is both a leader and a 

 follower may be entirely omitted. 



The power of a machine is not at all altered by 

 the size of the wheels, provided the proportions 

 to each other are the same. Formerly the wheels 

 of engines being mostly of wood, they were made 

 of a large size, on account of strength ; but now 

 that wheels are so easily made of cast iron, the 

 size of them is very much diminished, which has 

 the advantage of occupying much less room, with 

 greater durability. 



MOVING POWERS IN MACHINERY. 



The motion of machines must be excited and 

 kept up by some cause which is called the moving 

 power. 



These powers may be, the strength of men and 

 other animals ; or inanimate, as wind, water, steam, 

 gravity, or elasticity. 



In working machines, the choice of the moving 

 power must be regulated by convenience and 

 economical considerations, and also by the regu- 

 larity and intensity of its action. 



Weights and running water are most uniform in 

 their action, and steam is that which is capable of 

 the greatest intensity. The strength of animals is 

 the most unequal. 



; A -horse draws with the greatest advantage, when 

 the line of draught is not level with his breast; but 



