You remember that sunlight also helps the plants to sift the air, so that
they take from it the part that suits _them_, and leave behind the part
that suits _us_--that precious oxygen which is so necessary for all animal
life.
Then we must not forget the work done by the heat-waves. These are called
"dark," because they cannot be seen. They not only strike upon the land,
waking up the hidden seed, and warming it into life, but they are the great
water-carriers. When we were talking about the clouds we learnt that from
every wet place, as well as from the seas, lakes, and rivers, water is
constantly being drawn up, so that we can see it again in the fleecy clouds
which float across the sky, and again when it comes down in the showers
which water the earth--the tiny heat-waves are the messengers which perform
this work of evaporation.
When we were speaking about the world of water, we learnt that the moon is
the chief cause of the tides, by whose constant ebb and flow the ocean and
rivers are purified; in like manner the sun, by causing the winds to blow,
keeps the air fresh and pure; but this is a subject rather beyond us. We
can, however, remember that one more thing which the sun does for us is to
tell us the time. God gave him "to rule the day ... and to divide the light
from the darkness," and he marks how long our day is to be, "keeping time,"
as May's verse says, all the world over--for he is the great clock which
tells the hours and the days--a clock which never needs to be wound up,
and which we can trust, for it never goes wrong.
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