that we reckon the height of
mountains to the sea level only. Several of the craters on the Moon have
a diameter of 40 or 50--one of them even as much as 78--miles. Many
also have central cones, closely resembling those in our own volcanic
regions. In some cases the craters are filled nearly to the brim with
lava. The volcanoes seem, however, to be all extinct; and there is not a
single case in which we have conclusive evidence of any change in a
lunar mountain.

[Illustration: Fig. 50.--A group of Lunar Volcanoes.]

The Moon, being so much smaller than the earth, cooled, of course, much
more rapidly, and it is probable that these mountains are millions of
years old--much older than many of our mountain chains. Yet no one can
look at a map of the Moon without being struck with the very rugged
character of its mountain scenery. This is mainly due to the absence of
air and water. To these two mighty agencies, not merely "the
cloud-capped towers, the gorgeous palaces, the solemn temples," but the
very mountains themselves, are inevitable victims. Not merely storms and
hurricanes, but every gentle shower, every fall of snow, tends to soften
our scenery and lower the mountain peaks. These agencies are absent from
the Moon, and the mountains stand to-day just as they were formed
millions of years ago.

But though we find on our own globe (see, for instance, Fig. 21)
volcanic regions closely resembling those of the Moon, there are other
phenomena on the Moon's surface for which our earth presents as yet no
explanation. From Tycho, for instance, a crater 17,000 feet high and 50
miles across, a number of rays or streaks diverge, which for hundreds,
or in some cases two or three thousand, miles pass straight across
plains, craters, and mountains. The true nature of these streaks is not
yet understood.

THE SUN

The Sun is more than 400 times as distant as the Moon; a mighty glowing
globe, infinitely hotter than any earthly fiery furnace, 300,000 times
as heavy, and 1,000,000 times as large as the earth. Its diameter is
865,000 miles, and it revolves on its axis in between 25 and 26 days.
Its distance is 92,500,000 miles. And yet it is only a star, and by no
means one of the first magnitude.

The surface of the Sun is the seat of violent storms and tempests. From
it gigantic flames, consisting mainly of hydrogen, flicker and leap.
Professor Young describes one as being, when first observed, 40,000
miles high. Suddenly i