“But what explanation of these trails of light have been imagined?” asked Michel, “for I cannot believe that savants would ever stop short for want of explanation.”

“Yes,” answered Barbicane, “Herschel has uttered an opinion, but he does not affirm it.”

“Never mind; what is his opinion?”

“He thought that these rays must be streams of cold lava which shone when the sun struck them normally.”

“That may be true, but nothing is less certain.  However, if we pass nearer to Tycho we shall be in a better position to find out the cause of this radiation.”

“What do you think that plain is like, seen from the height we are at?” asked Michel.

“I don’t know,” answered Nicholl.

“Well, with all these pieces of lava, sharpened like spindles, it looks like ‘an immense game of spilikins,’ thrown down pell-mell.  We only want a hook to draw them up.”

“Be serious for once in your life,” said Barbicane.

“I will be serious,” replied Michel tranquilly, “and instead of spilikins let us say they are bones.  This plain would then be only an immense cemetery upon which would repose the immortal remains of a thousand distinct generations.  Do you like that comparison better?”

“One is as good as the other,” answered Barbicane.

“The devil!  You are difficult to please,” replied Michel.

“My worthy friend,” resumed the prosaic Barbicane, “it does not matter what it looks like when we don’t know what it is.”

“A good answer,” exclaimed Michel; “that will teach me to argue with savants.”

In the meantime the projectile went with almost uniform speed round the lunar disc.  It may be easily imagined that the travellers did not dream of taking a minute’s rest.  A fresh landscape lay before their eyes every instant.  About half-past one in the morning they caught a glimpse of the summit of another mountain.  Barbicane consulted his map, and recognised Eratosthenes.

It was a circular mountain 4,500 metres high, one of those amphitheatres so numerous upon the satellite.  Barbicane informed his friends of Kepler’s singular opinion upon the formation of these circles.  According to the celebrated mathematician, these crateriform cavities had been dug out by the hand of man.

“What for?” asked Nicholl.

“In order to preserve themselves from the ardour of the solar rays, which strike the moon during fifteen consecutive days.”

“The Selenites were not fools!” said Michel.

“It was a singular idea!” answered Nicholl.  “But it is probable that Kepler did not know the real dimensions of these circles, for digging them would have been giants’ labour, impracticable for Selenites.”

“Why so, if the weight on the surface of the moon is six times less than upon the surface of the earth?” said Michel.

“But if the Selenites are six times smaller?” replied Nicholl.