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. And he is a constant silent witness to us of the power and the goodness of God, as "day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language; their voice is not heard"--but "the heavens declare the glory of God ... in them hath He set a tabernacle for the sun." If, as we look at our watches, we are certain that men must have made them, how sure is it that God made this great time-keeper, light-giver, and life-sustainer--this mighty magnet that guides and controls the world of which it is the glorious centre! The sun "divides the light from the darkness" by being seen by us or hidden from our sight. If you watch, after the sun has risen in the morning--and you _can_ watch him in the winter, when you are often up before he is--you will see that he seems to climb the sky, always mounting higher and higher, until he is shining right above your head. Then, as the day goes on, and it gets towards afternoon, he seems to go down, down, until he sinks into the far away place where the earth and sky seem to meet, and we see him no more. It is while he is hidden from sight in the far west, behind that line which we call the horizon, that night wraps us in its deep shade; for the sun, the day-star is, gone.