valent to the velocity of the second gun at the time
it was fired plus the velocity produced by the explosion of its own
charge. In this way, by employing a series of guns, fired from each
other in succession, we can graduate the starting shock, and give the
bullet a final velocity sufficient to raise it against gravity, and the
resistance of the atmosphere, which grows less as it advances, and send
it away to the moon or some other distant orb."

[Footnote 2: _Engineering_, January 13th, 1893.]

_G_. "Your spit-fire mode of progression is well enough in theory, but
it strikes me as just a little complicated and risky. I, for one,
shouldn't care to emulate Elijah and shoot up to Heaven in that style."

_I_. "If it be all right in theory, it will be all right in practice.
However, instead of explosives we might employ compressed air to get the
required velocity. In the air-gun or cannon, as you probably know, a
quantity of air, compressed within a chamber of the breech, is allowed
suddenly to expand behind the bullet and eject it from the barrel. Now,
one might manage with a simple gun of this sort, provided it had a very
long barrel, and a series of air chambers at intervals from the breech
to the muzzle. Each of these chambers, beginning at the breech, could be
opened in turn as the bullet passed along the barrel, so that every
escaping jet of gas would give it an additional impulse."

_G._ (_with growing interest_). "That sounds neater. You might work the
chambers by electricity."

_I_. "We could even have an electric gun. Conceive a bobbin wound with
insulated wire in lieu of thread, and having the usual hole through the
axis of the frame. If a current of electricity be sent through the wire,
the bobbin will become a hollow magnet or 'solenoid,' and a plug of soft
iron placed at one end will be sucked into the hole. In this experiment
we have the germ of a solenoid cannon. The bobbin stands for the
gun-barrel, the plug for the bullet-car, and the magnetism for the
ejecting force. We can arrange the wire and current so as to draw the
plug or car right through the hole or barrel, and if we have a series of
solenoids end to end in one straight line, we can switch the current
through each in succession, and send the projectile with gathering
velocity through the interior of them all. In practice the barrel would
consist of a long straight tube, wide and strong enough to contain the
bullet-car without flexure, and