THE STEAM-ENGINE. 



465 



would cause the balls I to revolve with less speed, and the centrifugal force 

 produced by their circular motion would be diminished. This force being 

 thus no longer able fully to counteract their gravity, they would fall toward the 

 spindle, which would cause as, already explained, the throttle-valve to be 

 more fully opened. This would produce a more ample supply of steam to 

 the cylinder, by which the velocity of the machine would be restored to its 

 proper amount. 



The principle which renders the governor so perfect a regulator of the 

 velocity of the machine is difficult to be explained without having recourse to 

 the aid of the technical language of mathematical physics. As, however, this 

 instrument is of such great practical importance, and has attracted such general 

 admiration, it may be worth while here to attempt to render intelligible the 

 mechanical principles which govern its operation. Let S fig. 23, be the 



Fig. 23. 



B 



point of suspension of a common pendulum S P, and let P O P' be the arch 

 of its vibration, so that the ball P shall swing or vibrate alternately to the east 

 and to the west of the lowest point O, through the arches O P' and O P. It 

 is a property of such an instrument that, provided the arch in which it vibrates 

 be not considerable in magnitude, the time of its vibration will be the same 

 whether the arch be long or short. Thus, for example, if the pendulum, in- 

 stead of vibrating in the arch P P', vibrated in the arch p p' y the time which it 

 would take to perform its vibrations would be the same. If, however, the 

 magnitude of the arch of vibration be increased, then a variation will take place 

 in the time of vibration ; but unless the arch of vibration be considerably in- 

 creased, this variation will not be great. 



Now let it be supposed that while the pendulum P P' continues to vibrate 

 east and west through the arch P P', it shall receive such an impulse from 

 north and south as would, if it were not in a state of previous vibration, cause 

 it to vibrate between north and south, in an arch similar to the arch P P'. 

 This second vibration between north and south would not prevent the continu- 

 ance of the other vibration between east and west ; but the ball P would be at 

 the same time affected by both vibrations. While, in virtue of the vibration 

 from east to west, the ball would swing from P to P', it would, in virtue of the 

 other vibration, extend its motion toward the north to a distance from the line 

 W E equal to half a vibration, and will return from that distance again to the 

 position P'. While returning from P' to P, its second vibration will carry it 

 toward the south to an equal distance on the southern side of W E, and it will 

 return again to the position P. If the combination of these two motions or 



VOL,. II.— 30 



