The most careful studies of the
surface of this planet, under the best conditions, are those made
at the Lowell Observatory at Flagstaff, Arizona. Especially
wonderful is the system of so-called canals, first seen by
Schiaparelli, but mapped in great detail at Flagstaff. But the
nature and meaning of these mysterious lines are still to be
discovered. The result is that the question of the real nature of
the surface of Mars and of what we should see around us could we
land upon it and travel over it are still among the unsolved
problems of astronomy.
If this is the case with the nearest planets that we can study,
how is it with more distant ones? Jupiter is the only one of these
of the condition of whose surface we can claim to have definite
knowledge. But even this knowledge is meagre. The substance of
what we know is that its surface is surrounded by layers of what
look like dense clouds, through which nothing can certainly be
seen.
I have already spoken of the heat of the sun and its probable
origin. But the question of its heat, though the most important,
is not the only one that the sun offers us. What is the sun? When
we say that it is a very hot globe, more than a million times as
large as the earth, and hotter than any furnace that man can make,
so that literally "the elements melt with fervent heat" even at
its surface, while inside they are all vaporized, we have told the
most that we know as to what the sun really is.
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