ECCENTRIC. 



diminished, and the energy of the moving power, therefore, 

 mitigated. The undue increase of speed would thereby he 

 prevented. 



If, on the other hand, either hy an increase of the load, or a 

 diminished activity in the hoiler, the speed of the machine was 

 lessened, a corresponding diminution of velocity would take place 

 in the grooved wheel A B. This would cause the. balls 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 

 towards 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. 



45. The method of working the valves by means of pins pro- 

 jecting from the rod of the air pump has been in most cases super- 

 seded by an apparatus called an eccentric, by which the motion of 

 the axle of the fly-wheel is made to open and close the valves at 

 the proper times. 



An eccentric is a metallic circle attached to a revolving axle, so 

 that the centre of the circle shall not coincide with the centre 

 round which the axle revolves. Let us suppose that G (fig. 28) is 

 a square revolving" shaft. Let a circular plate of metal, B D, 



having its centre at c, have a square hole cut in it corresponding 

 to the shaft, G, and let the shaft, G, pass through this square 

 aperture, so that the circular plate, B D, shall be fastened upon 

 the shaft, and capable of revolving with it as the shaft revolves. 

 The centre, c, of the circular plate will be carried round the 

 centre, G, of the revolving shaft, and will describe round it a 



