[Footnote 1:  The Voyage a la Lune, by Jules Verne.]

I.  “Jules Verne has an original mind, and his ideas, though extravagant, are not without value.  Some of them have been realised, and it may be worth while to examine his notion of firing a shot from the earth to the moon.  The projectile, if I remember, was an aluminium shell in the shape of a conical bullet, and contained three men, a dog or two, and several fowls, together with provisions and instruments.  It was air tight, warmed and illuminated with coal gas, and the oxygen for breathing was got from chlorate of potash, while the carbonic 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