THE STEAM-ENGINE. 



473 



The method of working the valves by pins on the air-pump rod driving levers 

 connected with the valves has been, in almost all modern double-acting machines, 

 superseded by an apparatus called an eccentric, by which the motion of the 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. 26, is a square revolving shaft. Let a 



Fig. 26. 



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 B D will be carried round the centre G of the revolving shaft, and will 

 describe round it a circle, the radius of which will be the distance of the 

 centre C of the circular plate from the centre of the shaft. Such circular plate 

 so placed upon a shaft, and revolving with it, is an eccentric. 



Let E F be a metallic ring, formed of two semicircles of metal screwed 

 together at H, so as to be capable, by the adjustment of the screws, of having 

 the circular aperture formed by the ring enlarged and diminished within certain 

 small limits. Let this circular aperture be supposed to be equal to the magni- 

 tude of the eccentric B D. To the circular ring E F let an arm L M be at- 

 tached. If the ring E F be placed around the eccentric B D, and that the 

 screws H be so adjusted as to allow the eccentric B D to revolve within the 

 ring E F, then while the eccentric revolves, the ring not partaking of its 

 revolution, the arm L M will be alternately driven to the right and to the left, 

 by the motion of the centre C of the eccentric as it revolves round the centre 

 G of the axle. When the centre C of the eccentric is in the same horizontal 

 line with the centre G, and to the left of it, then the position of L M will be 

 that which is represented in fig. 26 ; but when, after half a revolution of the 

 main axle, the centre C of the eccentric is thrown on the other side of the 

 centre G, then the point M will be transferred to the right, to a distance equal 

 to twice the distance C G. Thus as the eccentric B D revolves within the 

 ring E F, that ring, together with the arm L M, will be alternately driven, 

 right and left, through a space equal to twice the distance between the centre 

 of the eccentric and the centre of the revolving shaft. 



If we suppose a notch formed at the extremity of the arm L M, which is 

 capable of embracing a lever N M, moveable on a pivot at N, the motion of 

 the eccentric would give to such a lever an alternate motion from right to left, 



