"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."