The travelers discussed the origin of these strange rays; but they could not determine their nature any more than terrestrial observers.

"But why," said Nicholl, "should not these rays be simply spurs of mountains which reflect more vividly the light of the sun?"

"No," replied Barbicane; "if it was so, under certain conditions of the moon, these ridges would cast shadows, and they do not cast any."

And indeed, these rays only appeared when the orb of day was in opposition to the moon, and disappeared as soon as its rays became oblique.

"But how have they endeavored to explain these lines of light?" asked Michel; "for I cannot believe that savants would ever be stranded for want of an explanation."

"Yes," replied Barbicane; "Herschel has put forward an opinion, but he did not venture to affirm it."

"Never mind. What was the opinion?"

"He thought that these rays might be streams of cooled lava which shone when the sun beat straight upon them. It may be so; but nothing can be less certain. Besides, if we pass nearer to Tycho, we shall be in a better position to find out the cause of this radiation."

"Do you know, my friends, what that plain, seen from the height we are at, resembles?" said Michel.

"No," replied Nicholl.

"Very well; with all those pieces of lava lengthened like rockets, it resembles an immense game of spelikans thrown pellmell. There wants but the hook to pull them out one by one."

"Do be serious," said Barbicane.

"Well, let us be serious," replied Michel quietly; "and instead of spelikans, let us put bones. This plain, would then be nothing but an immense cemetery, on which would repose the mortal remains of thousands of extinct generations. Do you prefer that high-flown comparison?"

"One is as good as the other," retorted Barbicane.

"My word, you are difficult to please," answered Michel.

"My worthy friend," continued the matter-of-fact Barbicane, "it matters but little what it resembles, when we do not know what it is."

"Well answered," exclaimed Michel. "That will teach me to reason with savants."

But the projectile continued to advance with almost uniform speed around the lunar disc. The travelers, we may easily imagine, did not dream of taking a moment's rest. Every minute changed the landscape which fled from beneath their gaze. About half past one o'clock in the morning, they caught a glimpse of the tops of another mountain. Barbicane, consulting his map, recognized Eratosthenes.

It was a ringed mountain nine thousand feet high, and one of those circles so numerous on this satellite. With regard to this, Barbicane related Kepler's singular opinion on the formation of circles. According to that celebrated mathematician, these crater-like cavities had been dug by the hand of man.

"For what purpose?" asked Nicholl.

"For a very natural one," replied Barbicane. "The Selenites might have undertaken these immense works and dug these enormous holes for a refuge and shield from the solar rays which beat upon them during fifteen consecutive days."

"The Selenites are not fools," said Michel.

"A singular idea," replied Nicholl; "but it is probable that Kepler did not know the true dimensions of these circles, for the digging of them would have been the work of giants quite impossible for the Selenites."

"Why? if weight on the moon's surface is six times less than on the earth?" said Michel.

"But if the Selenites are six times smaller?" retorted Nicholl.

"And if there are no Selenites?" added Barbicane.

This put an end to the discussion.

Soon Eratosthenes disappeared under the horizon without the projectile being sufficiently near to allow close observation. This mountain separated the Apennines from the Carpathians. In the lunar orography they have discerned some chains of mountains, which are chiefly distributed over the northern hemisphere. Some, however, occupy certain portions of the southern hemisphere also.

About two o'clock in the morning Barbicane found that they were above the twentieth lunar parallel.