depths of darkness and of once more
finding themselves, even if only for a few hours, in the cheerful
precincts illuminated by the genial light of the blessed Sun!

The ring of light, in the meantime, becoming brighter and brighter,
Barbican was not long in discovering and pointing out to his companions
the different mountains that lay around the Moon's south pole.

"There is _Leibnitz_ on your right," said he, "and on your left you can
easily see the peaks of _Doerfel_. Belonging rather to the Moon's dark
side than to her Earth side, they are visible to terrestrial astronomers
only when she is in her highest northern latitudes. Those faint peaks
beyond them that you can catch with such difficulty must be those of
_Newton_ and _Curtius_."

"How in the world can you tell?" asked Ardan.

"They are the highest mountains in the circumpolar regions," replied
Barbican. "They have been measured with the greatest care; _Newton_ is
23,000 feet high."

"More or less!" laughed Ardan. "What Delphic oracle says so?"

"Dear friend," replied Barbican quietly, "the visible mountains of the
Moon have been measured so carefully and so accurately that I should
hardly hesitate in affirming their altitude to be as well known as that
of Mont Blanc, or, at least, as those of the chief peaks in the
Himalayahs or the Rocky Mountain Range."

"I should like to know how people set about it," observed Ardan
incredulously.

"There are several well known methods of approaching this problem,"
replied Barbican; "and as these methods, though founded on different
principles, bring us constantly to the same result, we may pretty
safely conclude that our calculations are right. We have no time, just
now to draw diagrams, but, if I express myself clearly, you will no
doubt easily catch the general principle."

"Go ahead!" answered Ardan. "Anything but Algebra."

"We want no Algebra now," said Barbican, "It can't enable us to find
principles, though it certainly enables us to apply them. Well. The Sun
at a certain altitude shines on one side of a mountain and flings a
shadow on the other. The length of this shadow is easily found by means
of a telescope, whose object glass is provided with a micrometer. This
consists simply of two parallel spider threads, one of which is
stationary and the other movable. The Moon's real diameter being known
and occupying a certain space on the object glass, the exact space
occupied by the shadow can be easily as