acid
produced by the lungs and gas-burners was absorbed with caustic potash
to keep the air pure. This bullet-car was fired from a colossal
cast-iron gun founded in the sand. It was aimed at a point in the sky,
the zenith, in fact, where it would strike the moon four days later,
that is, after it had crossed the intervening space. The charge of
gun-cotton was calculated to give the projectile a velocity sufficient
to carry it past the 'dead-point,' where the gravity of the earth upon
it was just balanced by that of the moon, and enable it to fall towards
the moon for the rest of the way. The sudden shock of the discharge on
the car and its occupants was broken by means of spring buffers and
water pressure."

_G_. "The last arrangement was altogether inadequate."

_I_. "It was certainly a defect in the scheme."

_G_. "Besides, the initial velocity of the bullet to carry it beyond the
'dead-point,' was, I think, 12,000 yards a second, or something like
seven miles a second."

_I_. "His estimate was too high. An initial velocity of 9,000 yards, or
five miles a second, would carry a projectile beyond the sensible
attraction of the earth towards the moon, the planets, or anywhere; in
short, to an infinite distance. Indeed, a slightly lower velocity would
suffice in the case of the moon, owing to her attraction."

_G_. "But how are we to give the bullet that velocity? I believe the
highest velocity obtained from a single discharge of cordite, one of our
best explosives, was rather less than 4,000 feet, or only about
three-quarters of a mile per second. With such a velocity, the
projectile would simply rise to a great height and then fall back to the
ground."

_I_. "Both of these drawbacks can be overcome. We are not limited to a
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 equi