therefore, is this-- What thickness ought a cast-iron shell to
have in order not to weight more than 20,000 pounds? Our clever
secretary will soon enlighten us upon this point."

"Nothing easier." replied the worthy secretary of the committee;
and, rapidly tracing a few algebraical formulae upon paper,
among which _n_^2 and _x_^2 frequently appeared, he presently said:

"The sides will require a thickness of less than two inches."

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

"Clearly not!" replied the president.

"What is to be done, then?" said Elphinstone, with a puzzled air.

"Employ another metal instead of iron."

"Copper?" said Morgan.

"No! that would be too heavy. I have better than that to offer."

"What then?" asked the major.

"Aluminum!" replied Barbicane.

"Aluminum?" cried his three colleagues in chorus.

"Unquestionably, my friends. This valuable metal possesses the
whiteness of silver, the indestructibility of gold, the tenacity