"My dear major," replied Barbicane, "before discussing its weight, permit me to enumerate some of the marvels which our ancestors have achieved in this respect. I don't mean to pretend that the science of gunnery has not advanced, but it is as well to bear in mind that during the middle ages they obtained results more surprising, I will venture to say, than ours. For instance, during the siege of Constantinople by Mahomet II., in 1453, stone shot of 1900 lbs. weight were employed. At Malta, in the time of the knights, there was a gun of the fortress of St. Elmo which threw a projectile weighing 2500 lbs. And, now, what is the extent of what we have seen ourselves? Armstrong guns discharging shot of 500 lbs., and the Rodman guns projectiles of half a ton! It seems, then, that if projectiles have gained in range, they have lost far more in weight. Now, if we turn our efforts in that direction, we ought to arrive, with the progress of science, at ten times the weight of the shot of Mahomet II. and the Knights of Malta." Illustration: CANNON AT MALTA IN THE TIME OF THE KNIGHTS. "Clearly," replied the major; "but what metal do you calculate upon employing?" "Simply cast iron," said General Morgan. "But," interrupted the major, "since the weight of a shot is proportionate to its volume, an iron ball of nine feet in diameter would be of tremendous weight." "Yes, if it were solid, not if it were hollow." "Hollow? then it would be a shell?" "Yes, a shell," replied Barbicane; "decidedly it must be. A solid shot of 108 inches would weigh more than 200,000 lbs., a weight evidently far too great. Still, as we must reserve a certain stability for our projectile, I propose to give it a weight of 20,000 lbs." "What, then, will be the thickness of the sides?" asked the major. "If we follow the usual proportion," replied Morgan, "a diameter of 108 inches would require sides of two feet thickness, or less." "That would be too much," replied Barbicane; "for you will observe that the question is not that of a shot intended to pierce an iron plate: it will suffice, therefore, to give it sides strong enough to resist the pressure of the gas. The problem, therefore, is this—What thickness ought a cast-iron shell to have in order not to weigh more than 20,000 lbs.? 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 formulæ 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. "Aluminium!" replied Barbicane.