THE STEAM ENGINE. 



circle, the radius oi which will be the distance of the centre, c, of 

 the circular plate from the centre ol 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 magnitude of 

 the eccentric, B D. To the circular ring, E F, let an arm, L M, 

 be attached. If the ring, E r, be placed around the eccentric, 

 and the screws, H, be so adjusted as to allow the eccentric io 

 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 :\r 

 will be that which is represented in fig. 28 ; 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 revolves within the ring, 

 E F, that ring, together with the arm, L ar, 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, x 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, and vice versa. If we suppose 

 another lever, N o, connected with N" M, and at right angles to it, 

 forming what is called a bell-crank, then the alternate motion 

 received by M, from right to left, would give a corresponding 

 motion to the extremity, o, of the lever, N o, upwards and down- 

 wards. If this last point, o, were attached to a vertical arm or 

 shaft, it would impart to such arm or shaft an alternate motion 

 upwards and downwards, the extent of which would be regulated 

 by the length of the levers respectively. 



By such a contrivance the revolution of the shaft is made to 

 give an alternate vertical motion of any required extent to a 

 vertical shaft placed near the cylinder, which may be so connected 

 with the valves as to open and close them. Since the upward and 

 downward motion of this vertical shaft is governed by the alternate 

 motion of the centre, c, to the right and to the left of the cejitre, 

 40 



