CHAP, vi.] THE STEAM-ENGINE. 423 



to fear that the velocity may be always in excess or always in defect, 

 the fly-wheel is of no use, for it will itself acquire a too great or too 

 little velocity, and this excess in the first case may go on increasing 

 up to breaking point. The centrifugal force, which increases with the 

 square of the distance, would be the cause of this accident, and this 

 foreshadows the use of another kind of regulator, we mean the 

 centrifugal regulators, by the aid of which the engine itself regulates 

 its velocity in case of the steam leaving the boiler with excess of 

 pressure, or of the steam not arriving in sufficient quantity, and the 

 velocity of the motion diminishing. 



This apparatus consists of two metallic balls B, B, carried by two 

 rods OA, OA', jointed to a fixed point on a vertical axis. Two other 

 rods, jointed at A and A', are attached to a collar M, which clasps the 

 vertical axis and moves up and down along 

 it. The whole system receives also, through 

 the intervention of a pulley P, the motion 

 of rotation given to the driving-shaft of the 

 engine. Lastly, the collar, M is clasped by a 

 fork forming the end of one of the arms of a 

 lever I L. 



When the engine is working with its 

 required velocity the lever ML remains hori- 

 zontal. If the velocity increases, the centri- 

 fugal force lengthens" the distance of the 

 balls from the axis, the collar rises, and 



with it the arm of the lever IM. The other arm, IL, is lowered by 

 its turning about the point i. If, on the contrary, the velocity 

 diminishes, the centrifugal force is less ; the balls approach the 

 axis, which depresses the collar, and produces an opposite motion 

 in the lever. 



In the steam-pipe (the pipe supplying the cylinders with steam 

 from the boiler) is placed a valve consisting of a flat disc, moving 

 about an axis passing through its centre and lying in the plane of the 

 disc. To the extremities of the axis is rigidly attached a forked lever ; 

 when this is placed in one position the disc is at right angles to the 

 length of the steam-pipe and completely closes it; when moved from 

 this position it causes the disc to open the passage of the steam-pipe ; 

 and when the lever is in a position at right angles to the first, the 



