lained
by it, we have strong reason for believing that something of the kind
must have happened. When we come to speak of the starry heavens we shall
see that there are many masses of glowing gas which are nebulae of the
same sort, and which form an object-lesson in our own history.

We have spoken rather lightly of the nebula rotating and throwing off
planets; but we must not think of all this as having happened in a
short time. It is almost as impossible for the human mind to conceive
the ages required for such slow changes as to grasp the great gulfs of
space that separate us from the stars. We can only do it by comparison.
You know what a second is, and how the seconds race past without ceasing
day and night. It makes one giddy to picture the seconds there are in a
year; yet if each one of those seconds was a year in itself, what then?
That seems a stupendous time, but it is nothing compared with the time
needed to form a nebula into a planetary system. If we had five thousand
of such years, with every second in them a year, we should then only
have counted one billion real years, and billions must have passed since
the sun was a gaseous nebula filling the outermost bounds of our
system!

CHAPTER V

FOUR SMALL WORLDS

What must the sun appear to Mercury, who is so much nearer to him than
we are? To understand that we should have to imagine our sun increased
to eight or nine times his apparent size, and pouring out far greater
heat and light than anything that we have here, even in the tropics. It
was at first supposed that Mercury must have an extra thick covering of
clouds to protect him from this tremendous glare; but recent
observations tend to prove that, far from this, he is singularly free
from cloud. As this is so, no life as we know it could possibly exist on
Mercury.

His year--the time he takes to go round the sun and come back to the
same place again--is eighty-eight days, or about one-quarter of ours. As
his orbit is much more like an ellipse than a circle, it follows that he
is much nearer to the sun at one time than at another--in fact, when he
is nearest, the size of the sun must seem three and a half times
greater than when he is furthest away from it! Even at the best Mercury
is very difficult to observe, and what we can learn about him is not
much; but, as we have heard, his axis is supposed to be upright. If so
his seasons cannot depend on the bend toward or away from the sun, but
m