Since the date of the earlier double-acting engines, constructed by Boulton and Watt, a great variety of mechanical expedients have been practised for working the valves, by which the steam is admitted to and withdrawn from the [Pg225] cylinder. We shall here describe a few of these methods:— (128.) 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on the air-pump rod driving levers connected with the valves has been, in almost all modern double-acting machines, superseded by an apparatus called an _eccentric_, by which the motion of the axle of the fly-wheel is made to open and close the valves at the proper times. [Illustration: _Fig._ 45.] 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._ 45.), 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 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 [Pg226] 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 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._ 45.; 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.