Some of these furrows were as straight as if they had been cut by line, others were slightly curved through with edges still parallel.  Some crossed each other.  Some crossed craters.  Some furrowed the circular cavities, such as Posidonius or Petavius.  Some crossed the seas, notably the Sea of Serenity.

These accidents of Nature had naturally exercised the imagination of terrestrial astronomers.  The earliest observations did not discover these furrows.  Neither Hevelius, Cassini, La Hire, nor Herschel seems to have known them.  It was Schroeter who in 1789 first attracted the attention of savants to them.  Others followed who studied them, such as Pastorff, Gruithuysen, Boeer, and Moedler.  At present there are seventy-six; but though they have been counted, their nature has not yet been determined.  They are not fortifications certainly, anymore than they are beds of dried-up rivers, for water so light on the surface of the moon could not have dug such ditches, and there furrows often cross craters at a great elevation.

It must, however, be acknowledged that Michel Ardan had an idea, and that, without knowing it, he shared it with Julius Schmidt.

“Why,” said he, “may not these inexplicable appearances be simply phenomena of vegetation?”

“In what way do you mean?” asked Barbicane.

“Now do not be angry, worthy president,” answered Michel, “but may not these black lines be regular rows of trees?”

“Do you want to find some vegetation?” said Barbicane.

“I want to explain what you scientific men do not explain!  My hypothesis will at least explain why these furrows disappear, or seem to disappear, at regular epochs.”

“Why should they?”

“Because trees might become invisible when they lose their leaves, and visible when they grow again.”

“Your explanation is ingenious, old fellow,” answered Barbicane, “but it cannot be admitted.”

“Why?”

“Because it cannot be said to be any season on the surface of the moon, and, consequently, the phenomena of vegetation on the surface of the moon cannot be produced.”

In fact, the slight obliquity of the lunar axis keeps the sun there at an almost equal altitude under every latitude.  Above the equatorial regions the radiant orb almost invariably occupies the zenith, and hardly passes the limit of the horizon in the polar regions.  Therefore, in each region, according to its position, there reigns perpetual spring, summer, autumn, or winter, as in the planet Jupiter, whose axis is also slightly inclined upon its orbit.

The origin of these furrows is a difficult question to solve.  They are certainly posterior to the formation of the craters and amphitheatres, for several have crossed them, and broken their circular ramparts.  It may be that they are contemporary with the latest geographical epochs, and are only owing to the expansion of natural forces.