A.—­It may at all events be reduced.  The outline of the cam should be a parabola, so that the valve may be set in motion precisely as a falling body would be; but it will, nevertheless, be necessary that the roller on which the cam presses should be forced upward by a spring rather than by a counterweight, as there will thus be less inertia or momentum in the mass that has to be moved.

203. Q.—­An additional slide valve is sometimes used for cutting off the steam?

A.—­Yes, very frequently; and the slide valve is sometimes on the side or back of the valve casing, and sometimes on the back of the main or distributing valve, and moving with it.

204. Q.—­Are cams used in locomotive engines?

A.—­In locomotive engines the use of cams is inadmissible, and other expedients are employed, of which those contrived by Stephenson and by Cabrey operate on the principle of accomplishing the requisite variations of expansion by altering the throw of the slide valve.

205. Q.—­What is Stephenson’s arrangement?

[Illustration:  Fig. 35.]

A.—­Stephenson connects the ends of the forward and backward eccentric rods by a link with a curved slot in which a pin upon the end of the valve rod works.  By moving this link so as to bring the forward eccentric rod in the same line with the valve rod, the valve receives the motion due to that eccentric; whereas if the backward eccentric rod is brought in a line with the valve rod, the valve gets the motion proper for reversing, and if the link be so placed that the valve rod is midway between the two eccentric rods, the valve will remain nearly stationary.  This arrangement, which is now employed extensively, is what is termed “the link motion.”  It is represented in the annexed figure, fig. 35, where e is the valve rod, which is attached by a pin to an open curved link susceptible of being moved up and down by the bell-crank lever f’’ f’’, supported on the centre g, and acting on the links f, while the valve rod e remains in the same horizontal plane; d d’ are the eccentric rods, and the link is represented in its lowest position.  The dotted lines h’ h’’ show the position of the eccentric rods when the link is in its highest position, and l l’ when in mid position.

206. Q.—­What is Cabrey’s arrangement?

A.—­Mr. Cabrey makes his eccentric rod terminate in a pin which works into a straight slotted lever, furnished with jaws similar to the jaws on the eccentric rods of locomotives.  By raising the pin of the eccentric rod in this slot, the travel of the valve will be varied, and expansive action will be the result.

207. Q.—­What other forms of apparatus are there for working steam expansively?