From the Earth to the Moon HTML version
Chapter 8. History Of The Cannon
The resolutions passed at the last meeting produced a great effect out of doors. Timid
people took fright at the idea of a shot weighing 20,000 pounds being launched into
space; they asked what cannon could ever transmit a sufficient velocity to such a mighty
mass. The minutes of the second meeting were destined triumphantly to answer such
questions. The following evening the discussion was renewed.
"My dear colleagues," said Barbicane, without further preamble, "the subject now before
us is the construction of the engine, its length, its composition, and its weight. It is
probable that we shall end by giving it gigantic dimensions; but however great may be
the difficulties in the way, our mechanical genius will readily surmount them. Be good
enough, then, to give me your attention, and do not hesitate to make objections at the
close. I have no fear of them. The problem before us is how to communicate an initial
force of 12,000 yards per second to a shell of 108 inches in diameter, weighing 20,000
pounds. Now when a projectile is launched into space, what happens to it? It is acted
upon by three independent forces: the resistance of the air, the attraction of the earth, and
the force of impulsion with which it is endowed. Let us examine these three forces. The
resistance of the air is of little importance. The atmosphere of the earth does not exceed
forty miles. Now, with the given rapidity, the projectile will have traversed this in five
seconds, and the period is too brief for the resistance of the medium to be regarded
otherwise than as insignificant. Proceding, then, to the attraction of the earth, that is, the
weight of the shell, we know that this weight will diminish in the inverse ratio of the
square of the distance. When a body left to itself falls to the surface of the earth, it falls
five feet in the first second; and if the same body were removed 257,542 miles further
off, in other words, to the distance of the moon, its fall would be reduced to about half a
line in the first second. That is almost equivalent to a state of perfect rest. Our business,
then, is to overcome progressively this action of gravitation. The mode of accomplishing
that is by the force of impulsion."
"There's the difficulty," broke in the major.
"True," replied the president; "but we will overcome that, for the force of impulsion will
depend on the length of the engine and the powder employed, the latter being limited
only by the resisting power of the former. Our business, then, to-day is with the
dimensions of the cannon."
"Now, up to the present time," said Barbicane, "our longest guns have not exceeded
twenty-five feet in length. We shall therefore astonish the world by the dimensions we
shall be obliged to adopt. It must evidently be, then, a gun of great range, since the length
of the piece will increase the detention of the gas accumulated behind the projectile; but
there is no advantage in passing certain limits."
"Quite so," said the major. "What is the rule in such a case?"