“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; “decidely it must be. A solid shot of 108 inches would weigh more than 200,000 pounds, 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 pounds.” 

 “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 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 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 n2 and x2 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. 

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