The annexed cut represents Mr. Snodgrass's method of working expansively, with the ordinary slide valve. Upon the paddle shaft, and at the back of the crank, is fixed a spur wheel a, which gives motion to another wheel b, of similar dimensions; and which revolves upon a stout pivot or stud. This latter wheel corresponds to the ordinary eccentric, and has on its face a pin, to which is connected the eccentric rod c, which moves the valve shaft. On the face of the wheel a are two pins, which carry the antifriction rollers d d, that act alternately against the perpendicular lever c, to the top of which is jointed a rod f, that is connected to the governor valve g, placed in the pipe which conveys the steam to the slide case; h is a spring acting on the back of the lever, to force the lever forwards as the rollers d on the wheel a retire; and thus by the alternate action of the spring, and of the rollers d d, the governor valve is opened and closed twice during each revolution of the crank. The rod fmay be elongated or contracted at pleasure, by turning the nut, attached to the fork which connects it to the lever e, and by this means the steam may be cut off at any portion of the stroke, and the remainder of the stroke be performed by its expansion; k is a throttle valve to be regulated by hand; it is connected by a rod to a lever under the command of the pilot.
When conical valves are employed in lieu of slide valves, and receive their motion from an eccentric on the paddle shaft, a separate valve, moved by a separate cam, is necessary to work the steam expansively. This additional valve and cam may, however, be dispensed with, if, instead of the rocking shaft, a revolving shaft be substituted; carrying a separate cam for each valve; which can thus be made to open and close at that period of the stroke which is required to produce the best effect. The revolving shaft is put in motion by the paddle shaft, by means of suitable gearing. By this mode of working the valves, the action is rendered as rapid as if raised by tappets, and the concussions of the latter mode are avoided.