“Astonish us?” said Michel Ardan.

“But I believe that at the epoch when the moon was inhabited the nights and days did not last 354 hours!”

“Why so?” asked Nicholl quickly.

“Because it is very probable that then the moon’s movement of rotation on her axis was not equal to her movement of revolution, an equality which puts every point of the lunar disc under the action of the solar rays for fifteen days.”

“Agreed,” answered Nicholl; “but why should not these movements have been equal, since they are so actually?”

“Because that equality has only been determined by terrestrial attraction.  Now, how do we know that this attraction was powerful enough to influence the movements of the moon at the epoch the earth was still fluid?”

“True,” replied Nicholl; “and who can say that the moon has always been the earth’s satellite?”

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

Imagination began to wander in the indefinite field of hypotheses.  Barbicane wished to hold them in.

“Those,” said he, “are speculations too high, problems really insoluble.  Do not let us enter into them.  Let us only admit the insufficiency of primordial attraction, and then by the inequality of rotation and revolution days and nights could succeed each other upon the moon as they do upon the earth.  Besides, even under those conditions life was possible.”

“Then,” asked Michel Ardan, “humanity has quite disappeared from the moon?”

“Yes,” answered Barbicane, “after having, doubtless, existed for thousands of centuries.  Then gradually the atmosphere becoming rarefied, the disc will again be uninhabitable like the terrestrial globe will one day become by cooling.”

“By cooling?”

“Certainly,” answered Barbicane.  “As the interior fires became extinguished the incandescent matter was concentrated and the lunar disc became cool.  By degrees the consequences of this phenomenon came about—­the disappearance of organic beings and the disappearance of vegetation.  Soon the atmosphere became rarefied, and was probably drawn away by terrestrial attraction; the breathable air disappeared, and so did water by evaporation.  At that epoch the moon became uninhabitable, and was no longer inhabited.  It was a dead world like it is to-day.”

“And you say that the like fate is reserved for the earth?”

“Very probably.”

“But when?”

“When the cooling of its crust will have made it uninhabitable.”

“Has the time it will take our unfortunate globe to melt been calculated?”

“Certainly.”

“And you know the reason?”

“Perfectly.”

“Then tell us, sulky savant—­you make me boil with impatience.”

“Well, my worthy Michel,” answered Barbicane tranquilly, “it is well known what diminution of temperature the earth suffers in the lapse of a century.  Now, according to certain calculations, that average temperature will be brought down to zero after a period of 400,000 years!”