[THE MOON LENDS A HAND.]
BY CHARLES BARNARD.
If you drop a lump of sugar into a cupful of tea, or stir the tea about with a spoon, there will be little bubbles, floating on the surface. Watch these bubbles, and you will see that they soon slide off and gather along the edge of the cup. Boys in the first class in philosophy know what that means. It is the attraction of the cup. It is larger than the bubbles, and, as they are free to move about on the tea, they are attracted or pulled toward the sides of the cup.
If you lift the tea-cup, you find it is heavy. The great earth, that is millions of times larger than the cup, pulls it downward. We call it weight. We say the cup is pulled down by the attraction of gravitation.
Out of doors you can see the sun. It too has an attraction for the cup and for the whole round world and all it contains. It is bigger than our earth, and is pulling it toward itself. So strong is this attraction for the sun that everything that is lying loose on the earth would fly away if it were not that the world is so much nearer, and is attracting it the other way at the same time. There are some things that really start to go to the sun every day, but very fortunately they soon come back again.
Then there is the moon. She too is trying to pull everything toward herself. Poor Mrs. Moon! She is in an unfortunate position. She is pulled away toward the sun, and at the same time the earth attracts her this way. She wants to fly away and tumble into the sun, and she feels a great desire to fall down upon the world. She can't go both ways at once, so she contents herself with flying round the world once every day, and keeping us company in our journey round the sun.
The moon has her revenge on the earth. It pulls hard on the world all the time, and some of the things on the surface, that, like the bubbles in the tea-cup, are free to move, try every day to jump up to the moon. There is the air and all the water in the sea. They can move about, and whenever the moon passes overhead they move up as if to meet it. They can't go far, but they make a good start, and never seem tired of trying. If we could go up in a balloon to the top of the air we would probably find the air at one place piled up in a heap, as if it wanted to fly away to the moon if the earth would only give it a chance.
As it is not convenient for us to go up to the top of the air, we will go down to the beach to see how the water behaves when the moon goes by. No matter what time of the day or night you go to the sea-shore, you will find the water either rising up toward the moon or falling back again. It never seems to be discouraged, but as soon as it fails it starts again. You can not see it move, but if you put a stone at the edge of the water, and wait an hour or two, you will find the stone has been covered by the water or is left quite high and dry. It seems as if the whole of the great sea was forever slowly rising or falling, up and down, with a slow and solemn motion.
Any boy who lives by the shore knows that this is the tide. He knows that all his fun depends on this regular rising and falling of the tide. At high tide the fishing is good. At low tide the flats are bare, and the boys can dig clams or watch the long-legged plovers wading about in the shallow water. This curious rising and falling of the tide is caused by the attraction of the moon. The sun also helps, but in a lesser degree. How and why it all happens would take a long time to explain. We do not care for that just now, as the strange effects of the tides upon the land are more interesting.
I have already told you something of the way in which the sea and the waves are at work cutting out, tearing down, or building up the dry land on which we live. Perhaps you remember the stories of the walking beaches and the fight between the rivers and the sand-bars? We can now see what the moon has to do with this business.