the forces; it lies in the corresponding aggregation of the speeds. One
musket-ball will go, say a mile. It is not hard to increase the force of
muskets to a thousand, yet the thousand musket-balls will go no farther,
and no faster, than the one. You see, then, where our trouble lies. We
cannot readily add speed to speed, as we add force to force. My
discovery is simply the utilization of a principle which extorts an
increment of speed from each increment of power. But this is the
metaphysics of physics. Let us be practical or nothing.

"When you have walked forward, on a moving train, from the rear car,
toward the engine, did you ever think what you were really doing?"

"Why, yes, I have generally been going to the smoking-car to have a
cigar."

"Tut, tut--not that! I mean, did it ever occur to you on such an
occasion, that absolutely you were moving faster than the train? The
train passes the telegraph poles at the rate of thirty miles an hour,
say. You walk toward the smoking-car at the rate of four miles an hour.
Then _you_ pass the telegraph poles at the rate of thirty-four miles.
Your absolute speed is the speed of the engine, plus the speed of your
own locomotion. Do you follow me?"

I began to get an inkling of his meaning, and told him so.

"Very well. Let us advance a step. Your addition to the speed of the
engine is trivial, and the space in which you can exercise it, limited.
Now suppose two stations, A and B, two miles distant by the track.
Imagine a train of platform cars, the last car resting at station A. The
train is a mile long, say. The engine is therefore within a mile of
station B. Say the train can move a mile in ten minutes. The last car,
having two miles to go, would reach B in twenty minutes, but the engine,
a mile ahead, would get there in ten. You jump on the last car, at A, in
a prodigious hurry to reach Abscissa, who is at B. If you stay on the
last car it will be twenty long minutes before you see her. But the
engine reaches B and the fair lady in ten. You will be a stupid
reasoner, and an indifferent lover, if you don't put for the engine over
those platform cars, as fast as your legs will carry you. You can run a
mile, the length of the train, in ten minutes. Therefore, you reach
Abscissa when the engine does, or in ten minutes--ten minutes sooner
than if you had lazily sat down upon the rear car and talked politics
with the brakeman. You have diminished the time by one half. You hav