"Cast-iron!" exclaimed J.T. Maston disdainfully, "that's very common for
a bullet destined to go to the moon."

"Do not let us exaggerate, my honourable friend," answered Morgan;
"cast-iron will be sufficient."

"Then," replied Major Elphinstone, "as the weight of the projectile is
in proportion to its volume, a cast-iron bullet, measuring nine feet in
diameter, will still be frightfully heavy."

"Yes, if it be solid, but not if it be hollow," said Barbicane.

"Hollow!--then it will be an obus?"

"In which we can put despatches," replied J.T. Maston, "and specimens of
our terrestrial productions."

"Yes, an obus," answered Barbicane; "that is what it must be; a solid
bullet of 108 inches would weigh more than 200,000 lbs., a weight
evidently too great; however, as it is necessary to give the projectile
a certain stability, I propose to give it a weight of 20,000 lbs."

"What will be the thickness of the metal?" asked the major.

"If we follow the usual proportions," replied Morgan, "a diameter of 800
inches demands sides two feet thick at least."

"That would be much too thick," answered Barbicane; "we do not want a
projectile to pierce armour-plate; it only needs sides strong enough to