ll 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. Of course we know a great deal about the spots, the
rotation of the sun on its axis, the materials of which it is composed,
and how its surroundings look during a total eclipse. But all this does
not answer our question. There are several mysteries which ingenious
men have tried to explain, but they cannot prove their explanations to
be correct. One is the cause and nature of the spots. Another is that
the shining surface of the sun, the "photosphere," as it is technically
called, seems so calm and quiet while forces are acting within it of a
magnitude quite beyond our conception. Flames in which our earth and
everything on it would be engulfed like a boy's marble in a
blacksmith's forge are continually shooting up to a height of tens of
thousands of miles. One would suppose that internal forces capable of
doing this would break the surface up into billows of fire a thousand
miles high; but we see nothing of the kind. The surface of the sun
seems almost as placid as a lake.

Yet another mystery is the corona of the sun. This is something we
should never have known to exist if the sun were not sometimes totally
eclipsed by the dark body of the moon. On these rare occasions the sun
is seen to be surrounded by a halo of soft, white light, sending out
rays in various directions to great distances. This halo is called the