"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 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 of iron, the fusibility of copper, the lightness of glass. It is easily wrought, is very widely distributed, forming the base of most of the rocks, is three times lighter than iron, and seems to have been created for the express purpose of furnishing us with the material for our projectile."

"But, my dear president," said the major, "is not the cost price of aluminum extremely high?"

"It was so at its first discovery, but it has fallen now to nine dollars a pound."

"But still, nine dollars a pound!" replied the major, who was not willing readily to give in; "even that is an enormous price."

"Undoubtedly, my dear major; but not beyond our reach."

"What will the projectile weigh then?" asked Morgan.

"Here is the result of my calculations," replied Barbicane. "A shot of 108 inches in diameter, and twelve inches in thickness, would weigh, in cast-iron, 67,440 pounds; cast in aluminum, its weight will be reduced to 19,250 pounds."

"Capital!" cried the major; "but do you know that, at nine dollars a pound, this projectile will cost----"

"One hundred and seventy-three thousand and fifty dollars ($173,050). I know it quite well. But fear not, my friends; the money will not be wanting for our enterprise. I will answer for it. Now what say you to aluminum, gentlemen?"

"Adopted!" replied the three members of the committee. So ended the first meeting. The question of the projectile was definitely settled.

CHAPTER VII

HISTORY OF THE CANNON

The resolutions passed at the last meeting produced a great effect out of doors. Timid people took fright at the idea of a shot weighing 20,000 pounds being launched into space; they asked what cannon could ever transmit a sufficient velocity to such a mighty mass. The minutes of the second meeting were destined triumphantly to answer such questions. The following evening the discussion was renewed.

"My dear colleagues," said Barbicane, without further preamble, "the subject now before us is the construction of the engine, its length, its composition, and its weight. It is probable that we shall end by giving it gigantic dimensions; but however great may be the difficulties in the way, our mechanical genius will readily surmount them. Be good enough, then, to give me your attention, and do not hesitate to make objections at the close. I have no fear of them. The problem before us is how to communicate an initial force of 12,000 yards per second to a shell of 108 inches in diameter, weighing 20,000 pounds. Now when a projectile is launched into space, what happens to it? It is acted upon by three independent forces: the resistance of the air, the attraction of the earth, and the force of impulsion with which it is endowed. Let us examine these three forces.

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