loyed a spring formed of a great number of
discs of India rubber to keep the roller against the cam, but a few brass
discs require to be interposed to prevent the India rubber discs from being
worn in the central hole.

202. _Q._--May not the percussion incident to the action of a cam at a high
speed, when the roller is not kept up to the face by springs, be obviated
by giving a suitable configuration to the cam itself?

_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 p