The valve gear is of the Stephenson shifting-link pattern (see fig. 27),
a simple and dependable motion used extensively in this country between
about 1850 and 1900. The author believes that this is the original valve
gear of the _Pioneer_, since the first mention (1854) in the _Annual
Report_ of the Cumberland Valley Railroad of the style of valve gear
used by each engine, states that the _Pioneer_ was equipped with a
shifting-link motion. Assuming this to be the original valve gear of the
_Pioneer_, it must be regarded as an early application, because the
Stephenson motion was just being introduced into American locomotive
practice in the early 1850's. Four eccentrics drive the motion; two are
for forward motion and two for reverse. The link is split and made of
two curved pieces. The rocker is fabricated of several forged pieces
keyed and bolted together. On better made engines the rocker would be a
one-piece forging. The lower arm of each rocker is curiously shaped,
made with a slot so that the link block may be adjusted. Generally, the
only adjustment possible was effected by varying the length of the valve
stem by the adjusting nuts provided. A simple weight and lever attached
to the reversing shaft serve as a counterbalance for the links and thus
assist the engineer in shifting the valve motion. There are eight
positions on the quadrant of the reversing lever.
[Illustration: Figure 29.--"PIONEER" on exhibit in old Arts and
Industries building, showing the tank and backhead. (Smithsonian photo
48069E.)]