GOVERNORS. 



107 



governor could be altered. By altering these adjustments the regulation could 

 be made more and more rapid, till at last a dancing motion of the governor, 

 accompanied with a jerking motion of the main shaft, shewed that an alteration 

 had taken place among the impossible roots of the equation. 



I shall consider three kinds of governors, corresponding to the three kinds 

 of moderators already referred to. 



In the first kind, the centrifugal piece has a constant distance from the 

 axis of motion, but its pressure on a surface on which it rubs varies when the 

 velocity varies. In the moderator this friction is itself the retarding force. In 

 the governor this surface is made moveable about the axis, and the friction 

 tends to move it ; and this motion is made to act on a break to retard the 

 machine. A constant force acts on the moveable wheel in the opposite direction 

 to that of the friction, which takes off the break when the friction is less than 

 a given quantity. 



Mr Jenkin's governor is on this principle. It has the advantage that the 

 centrifugal piece does not change its position, and that its pressure is always 

 the same function of the velocity. It has the disadvantage that the normal 

 velocity depends in some degree on the coefficient of sliding friction between 

 two surfaces which cannot be kept always in the same condition. 



In the second kind of governor, the centrifugal piece is free to move further 

 from the axis, but is restrained by a force the intensity of which varies with 

 the position of the centrifugal piece in such a way that, if the velocity of 

 rotation has the normal value, the centrifugal piece will be in equilibrium in 

 every position. If the velocity is greater or less than the normal velocity, the 

 centrifugal piece will fly out or fall in without any limit except the limits of 

 motion of the piece. But a break is arranged so that it is made more or less 

 powerful according to the distance of the centrifugal piece from the axis, and 

 thus the oscillations of the centrifugal piece are restrained within narrow limits. 



Governors have been constructed on this principle by Sir W. Thomson and 

 by M. Foucault. In the first, the force restraining the centrifugal piece is that of 

 a spring acting between a point of the centrifugal piece and a fixed point at 

 a considerable distance, and the break is a friction-break worked by the reaction 

 of the spring on the fixed point. 



In M. Foucault's arrangement, the force acting on the centrifugal piece is 

 the weight of the balls acting downward, and an upward force produced by 



142 



