Light, like heat, can diffuse itself in the air; hence an equality between the influences which no longer exists, now that atmosphere has almost entirely disappeared. And now I am going to astonish you."

"Astonish us?" said Michel Ardan.

"I firmly believe that at the period when the moon was inhabited, the nights and days did not last 354 hours!"

"And why?" asked Nicholl quickly.

"Because most probably then the rotary motion of the moon upon her axis was not equal to her revolution, an equality which presents each part of her disc during fifteen days to the action of the solar rays."

"Granted," replied Nicholl, "but why should not these two motions have been equal, as they are really so?"

"Because that equality has only been determined by terrestrial attraction. And who can say that this attraction was powerful enough to alter the motion of the moon at that period when the earth was still fluid?"

"Just so," replied Nicholl; "and who can say that the moon has always been a satellite of the earth?"

"And who can say," exclaimed Michel Ardan, "that the moon did not exist before the earth?"

Their imaginations carried them away into an indefinite field of hypothesis. Barbicane sought to restrain them.

"Those speculations are too high," said he; "problems utterly insoluble. Do not let us enter upon them. Let us only admit the insufficiency of the primordial attraction; and then by the inequality of the two motions of rotation and revolution, the days and nights could have succeeded each other on the moon as they succeed each other on the earth. Besides, even without these conditions, life was possible."

"And so," asked Michel Ardan, "humanity has disappeared from the moon?"

"Yes," replied Barbicane, "after having doubtless remained persistently for millions of centuries; by degrees the atmosphere becoming rarefied, the disc became uninhabitable, as the terrestrial globe will one day become by cooling."

"By cooling?"

"Certainly," replied Barbicane; "as the internal fires became extinguished, and the incandescent matter concentrated itself, the lunar crust cooled. By degrees the consequences of these phenomena showed themselves in the disappearance of organized beings, and by the disappearance of vegetation. Soon the atmosphere was rarefied, probably withdrawn by terrestrial attraction; then aerial departure of respirable air, and disappearance of water by means of evaporation. At this period the moon becoming uninhabitable, was no longer inhabited. It was a dead world, such as we see it to-day."

"And you say that the same fate is in store for the earth?"

"Most probably."

"But when?"

"When the cooling of its crust shall have made it uninhabitable."

"And have they calculated the time which our unfortunate sphere will take to cool?"

"Certainly."

"And you know these calculations?"

"Perfectly."

"But speak, then, my clumsy savant," exclaimed Michel Ardan, "for you make me boil with impatience!"

"Very well, my good Michel," replied Barbicane quietly; "we know what diminution of temperature the earth undergoes in the lapse of a century. And according to certain calculations, this mean temperature will after a period of 400,000 years, be brought down to zero!"

"Four hundred thousand years!" exclaimed Michel. "Ah! I breathe again. Really I was frightened to hear you; I imagined that we had not more than 50,000 years to live."

Barbicane and Nicholl could not help laughing at their companion's uneasiness. Then Nicholl, who wished to end the discussion, put the second question, which had just been considered again.

"Has the moon been inhabited?" he asked.

The answer was unanimously in the affirmative. But during this discussion, fruitful in somewhat hazardous theories, the projectile was rapidly leaving the moon: the lineaments faded away from the travelers' eyes, mountains were confused in the distance; and of all the wonderful, strange, and fantastical form of the earth's satellite, there soon remained nothing but the imperishable remembrance.