g-tester acts on a human being; the
cylinders draw out the steam from the boiler, requiring a roaring fire
to make the vapour rapidly enough and keep up the pressure.

Though the engineer seemed to be taking it easily enough with his hand
resting lightly on the reversing-lever, his body at rest, the fireman
was kept on the jump. If he was not shovelling coal he was looking ahead
for signals (for many roads require him to verify the engineer), or
adjusting the valves that admitted steam to the train-pipes and heated
the cars, or else, noticing that the water in the boiler was getting
low--and this is one of his greatest responsibilities, which, however,
the engineer sometimes shares--he turned on the steam in the injector,
which forced the water against the pressure into the boiler. All these
things he has to do repeatedly even on a short run.

The engineer--or "runner," as he is called by his fellows--has much to
do also, and has infinitely greater responsibility. On him depends the
safety and the comfort of the passengers to a large degree; he must
nurse his engine to produce the greatest speed at the least cost of
coal, and he must round the curves, climb the grades, and make the
slow-downs and stops so gradually that the passengers will not be
disturbed.

To the outsider who rides in a locomotive-cab for the first time it
seems as if the engineer settles down to his real work with a sigh of
relief when the limits of the city have been passed; for in the towns
there are many signals to be watched, many crossings to be looked out
for, and a multitude of moving trains, snorting engines, and tooting
whistles to distract one's attention. The "runner," however, seemed not
to mind it at all. He pulled on his cap a little more firmly, and, after
glancing at his watch, reached out for the throttle handle. A very
little pull satisfied him, and though the increase in speed was hardly
perceptible, the more rapid exhaust told the story of faster movement.
As the train sped on, the engineer moved the reversing-lever notch by
notch nearer the centre of the guide. This adjusted the "link-motion"
mechanism, which is operated by the driving-axle, and cut off the steam
entering the cylinders in such a way that it expanded more fully and
economically, thus saving fuel without loss of power.

When a station was reached, when a "caution" signal was displayed, or
whenever any one of the hundred or more things occurred that might
requir