sun, and it is
caused by the earth's shadow falling upon the moon; and that of the sun
at new moon, when she is nearest to him, and it is caused by the solid
body of the moon coming between us and the sun.

[Illustration: AN ECLIPSE OF THE SUN.]

Besides giving us light by night, the moon serves other important
purposes, and the most important of all is the raising of the tides.
Without the rising of the sea twice in every day and night our coasts
would become foul and unwholesome, for all the dead fish and rotting
stuff lying on the beach would poison the air. The sea tides scour our
coasts day by day with never-ceasing energy, and they send a great
breath of freshness up our large rivers to delight many people far
inland. The moon does most of this work, though she is a little helped
by the sun. The reason of this is that the moon is so near to the earth
that, though her pull is a comparatively small one, it is very strongly
felt. She cannot displace the actual surface to any great extent, as it
is so solid; but when it comes to the water she can and does displace
that, so that the water rises up in answer to her pull, and as the earth
turns round the raised-up water lags behind, reaching backward toward
the moon, and is drawn up on the beach, and makes high tide. But it is
stopped there, and meantime, by reason of the earth's movement, the moon
is left far behind, and pulls the water to itself further on, when the
first high tide relapses and falls down again. At length the moon gets
round to quite the opposite side of the earth to that where she began,
and there she makes a high tide too; but as she draws the water to
herself she draws also the solid earth beneath the water to her in some
degree, and so pulls it away from the place where the first high tide
occurred, leaving the water there deeper than before, and so causing a
secondary high tide.

[Illustration: THE MOON RAISING THE TIDES.]

The sun has some influence on the tides too, and when moon and sun are
in the same line, as at full and new moon, then the tides are highest,
and are called spring tides; but when they pull in different directions,
as when it is half-moon, then the tides are lowest and are called neap
tides.

CHAPTER IV

THE EARTH'S BROTHERS AND SISTER

The earth is not the only world that, poised in space, swings around the
sun. It is one of a family called the Solar System, which means the
system controlled and governed by th