single discharge. Dr. S. Tolver Preston, the well-known writer on
molecular science, has pointed out that a very high velocity can be got
by the use of a compound gun, or, in other words, a gun which fires
another gun as a projectile.[2] Imagine a first gun of enormous
dimensions loaded with a smaller gun, which in turn is loaded with the
bullet. The discharge of the first gun shoots the second gun into the
air, with a certain velocity. If, now, the second gun, at the instant it
leaves the muzzle of the first, is fired automatically, say by
utilising the first discharge to press a spring which can react on a
hammer or needle, the bullet will acquire a velocity due to both
discharges, and equivalent 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