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.