"That is the problem indeed," answered Major Elphinstone.

"When a projectile is hurled into space," resumed Barbicane, "what
happens? It is acted upon by three independent forces, the resistance of
the medium, the attraction of the earth, and the force of impulsion with
which it is animated. Let us examine these three forces. The resistance
of the medium--that is to say, the resistance of the air--is of little
importance. In fact, the terrestrial atmosphere is only forty miles
deep. With a rapidity of 12,000 yards the projectile will cross that in
five seconds, and this time will be short enough to make the resistance
of the medium insignificant. Let us now pass to the attraction of the
earth--that is to say, to the weight of the projectile. We know that
that weight diminishes in an inverse ratio to the square of
distances--in fact, this is what physics teach us: when a body left to
itself falls on the surface of the earth, it falls 15 feet in the first
second, and if the same body had to fall 257,542 miles--that is to say,
the distance between the earth and the moon--its fall would be reduced
to half a line in the first second. That is almost equivalent to
immobility. The question is, therefore, how progressively to overcome
this law of gravitation. How shall we do it? By the force of impulsion?"

"That is the difficulty," answered the major.

"That is it indeed," replied the president. "But we shall triumph over
it, for this force of impulsion we want depends on the length of the
engine and the quantity of powder employed, the one only being limited
by the resistance of the other. Let us occupy ourselves, therefore,
to-day with the dimensions to be given to the cannon. It is quite
understood that we can make it, as large as we like, seeing it will not
have to be moved."