422 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



two points 0, o. The oscillation of the beam, that this result may 

 hold, must not exceed the limit of 20 on one side or the other of the 

 horizontal. The middle point of the side EC has the same properties 

 as D ; and this fact is also made use of in Woolff s engines, where the 

 pistons of the two cylinders must move together. 



It is to be understood that the arrangement just described is 

 repeated on the other side of the same end of the beam, so that in 

 reality the piston-rod is jointed to a horizontal axis, which passes 

 though the double point D. 



V. REGULATORS. 



It will be seen, on referring to Fig. 299, that on the axis, moved 

 by the system of connecting-rod and crank described above, is 

 mounted a large wheel v, generally of cast iron, which is called the 

 fly-wheel. This piece, which is found in all driving engines, is for 

 regulating the motion. 



In a driving engine, the velocity is subject to variations, which 

 may depend either on the motive force itself, that is, on the steam 

 which comes from the generator more or less abundantly, and 

 possessing a pressure of greater or less degree, or on the employ- 

 ment of the force in the factory where the engine is set up. It is 

 easy to see that it is advisable to have these variations kept 

 within narrow limits, which may be accomplished in various ways ; 

 and one of these ways is the employment of fly-wheels, which 

 increase the mass of the movable parts of the engine. When there 

 is excess of velocity the mass of the fly-wheel absorbs the excess of 

 motive power from the form of moving force, and restores it to the 

 various parts of the engine when the motion relaxes. The fly-wheel 

 is made both of great weight and large diameter, and the greater part 

 of its mass is concentrated in the rim that forms its circumference. 



The dimensions and weight of the fly-wheels are calculated accord- 

 ing to the power of the engine, and the greater or less irregularity of 

 the motion, and the resistance to be overcome. 



The use of a fly-wheel to regulate the motion of a steam-engine 

 does not fulfil its object unless the velocity is sometimes greater and 

 sometimes less than the normal velocity. But if there be any reason 



