250 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



bar in equilibrium at the desired speed. The motion of the 

 governor shown is over from left to right. When starting up, 

 the positions of the governor parts are as shown. In this 

 position the eccentric pin travels in its greatest path, giving 

 the valve its widest opening and latest cut-off. As the speed 

 increases, centrifugal force, acting on the weighted end of 

 inertia bar, moves it backwards towards the left hand stop, 

 thereby reducing the travel of valve and cutting oft' steam 

 earlier. This backward movement of the inertia bar is con- 

 tinued until the travel of the valve reduces the cut-oft to just 

 what is necessary to keep the engine rotating at the proper 

 speed to give even balance between centrifugal force acting 

 on the inertia bar, and the opposing tension of the spring. 



Fig. 158. The McEwen Engine. 



Should a load be suddenly applied to the engine there is a 

 check to the speed of the governor wheel, while the inertia 

 bar, by reason of its momentum, rotates forward sufficiently 

 to increase valve travel, and give late enough cut-oft to carry 

 the load. Should the load be thrown oft, the governor wheel 

 tends to increase its speed, thus causing the inertia bar to 

 swing backward and reduce travel and cut-oft until equili- 

 brium is established as before. It will be seen that in this 

 way the speed of rotation under a steady load depends entirely 

 on the equilibrium between the centrifugal forces on the in- 

 ertia bar and the tension of the governor spring, while the 

 actual movement of the governor parts is effected by the in- 

 ertia of the weighted end. If the weights in the inertia bar 



