because, if they were, under
certain conditions of solar illumination they should project
_shadows_--a thing which they have never been known to do under any
circumstances whatever. In fact, it is only during the period of the
full Moon that these streaks are seen at all; as soon as the sun's rays
become oblique, they disappear altogether--a proof that their appearance
is due altogether to peculiar advantages in their surface for the
reflection of light."

"Dear boys, will you allow me to give my little guess on the subject?"
asked Ardan.

His companions were profuse in expressing their desire to hear it.

"Well then," he resumed, "seeing that these bright streaks invariably
start from a certain point to radiate in all directions, why not suppose
them to be streams of lava issuing from the crater and flowing down the
mountain side until they cooled?"

"Such a supposition or something like it has been put forth by
Herschel," replied Barbican; "but your own sense will convince you that
it is quite untenable when you consider that lava, however hot and
liquid it may be at the commencement of its journey, cannot flow on for
hundreds of miles, up hills, across ravines, and over plains, all the
time in streams of almost exactly equal width."

"That theory of yours holds no more water than mine, Ardan," observed
M'Nicholl.

"Correct, Captain," replied the Frenchman; "Barbican has a trick of
knocking the bottom out of every weaker vessel. But let us hear what he
has to say on the subject himself. What is your theory. Barbican?"

"My theory," said Barbican, "is pretty much the same as that lately
presented by an English astronomer, Nasmyth, who has devoted much study
and reflection to lunar matters. Of course, I only formulate my theory,
I don't affirm it. These streaks are cracks, made in the Moon's surface
by cooling or by shrinkage, through which volcanic matter has been
forced up by internal pressure. The sinking ice of a frozen lake, when
meeting with some sharp pointed rock, cracks in a radiating manner:
every one of its fissures then admits the water, which immediately
spreads laterally over the ice pretty much as the lava spreads itself
over the lunar surface. This theory accounts for the radiating nature of
the streaks, their great and nearly equal thickness, their immense
length, their inability to cast a shadow, and their invisibility at any
time except at or near the Full Moon. Still it is nothing but a theor