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.