ght 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
resist the pressure of the powder-gas. This, therefore, is the
problem:--What thickness ought an iron obus to have in order to weigh
only 20,000 lbs.? Our clever calculator, Mr. Maston, will tell us at
once."

"Nothing is easier," replied the honourable secretary.

So saying, he traced some algebraical signs on the paper, amongst which
n^2 and x^2 frequently appeared. He even seemed to extract from them a
certain cubic root, and said--

"The sides must be hardly two inches thick."

"Will that be sufficient?" asked the major doubtfully.

"No," answered the president, "certainly not."

"Then what must be done?" resumed Elphinstone, looking puzzled.

"We must use another metal instead of cast-iron."

"Brass?" suggested Morgan.

"No; that is too heavy too, and I have something better than that to
propose."

"What?" asked the major.

"Aluminium," answered Barbicane.

"Aluminium!" cried all the three colleagues of the president.

"Certainly, my friends. You know that an illustrious French chemist,
Henry St. Claire Deville, succeeded in 1854 in obtaining aluminium in a
compact mass. This precious metal possesses the whiteness of silver, the
indestructibility of gold, the tenacity of iron, the fusibility of
copper, the lightness of glass; it is easily wrought, and is very widely
distributed in nature, as aluminium forms the basis of most rocks; it is
three times lighter than iron, and seems to have been created expressly
to furnish us with the material for our projectile!"