ct is still a
matter of uncertainty, but it is probably composed of a kind of dust, as
the faint spectrum it affords is of a continuous type. A view of the
zodiacal light is shown in Fig. 22.

In all directions the sun pours forth, with the most prodigal
liberality, its torrents of light and of heat. The earth can only grasp
the merest fraction, less than the 2,000,000,000th part of the whole.
Our fellow planets and the moon also intercept a trifle; but how small
is the portion of the mighty flood which they can utilise! The sip that
a flying swallow takes from a river is as far from exhausting the water
in the river as are the planets from using all the heat which streams
from the sun.

The sun's gracious beams supply the magic power that enables the corn to
grow and ripen. It is the heat of the sun which raises water from the
ocean in the form of vapour, and then sends down that vapour as rain to
refresh the earth and to fill the rivers which bear our ships down to
the ocean. It is the heat of the sun beating on the large continents
which gives rise to the breezes and winds that waft our vessels across
the deep; and when on a winter's evening we draw around the fire and
feel its invigorating rays, we are only enjoying sunbeams which shone on
the earth countless ages ago. The heat in those ancient sunbeams
developed the mighty vegetation of the coal period, and in the form of
coal that heat has slumbered for millions of years, till we now call it
again into activity. It is the power of the sun stored up in coal that
urges on our steam-engines. It is the light of the sun stored up in coal
that beams from every gaslight in our cities.

For the power to live and move, for the plenty with which we are
surrounded, for the beauty with which nature is adorned, we are
immediately indebted to one body in the countless hosts of space, and
that body is the sun.

[Illustration: Fig. 22.--The Zodiacal Light in 1874.]

CHAPTER III.

THE MOON.

The Moon and the Tides--The Use of the Moon in Navigation--The
Changes of the Moon--The Moon and the Poets--Whence the Light of
the Moon?--Sizes of the Earth and the Moon--Weight of the
Moon--Changes in Apparent Size--Variations in its
Distance--Influence of the Earth on the Moon--The Path of the
Moon--Explanation of the Moon's Phases--Lunar Eclipses--Eclipses of
the Sun, how produced--Visibility of the Moon in a Total
Eclipse--How Ecli