Right where you sit reading, perhaps, the land used to be buried two miles deep beneath rocks which have been worn away by wind and rain and by rivers which vanished long ago. Everything has been so changed that if the old scenery should be put back you would be lost right on the home farm. WHERE YOU CAN JUMP ACROSS THE MISSISSIPPI Wrinkles in the earth and in the mountainsides make the first troughs for the streamlets and the rivers, and then the running water itself digs these natural channels deeper. Many rivers begin as streamlets flowing out of springs. The great Mississippi began as a baby, just like the rest of us. You can jump across it still if you go up to its source. Springs not only start rivers in life but go on feeding them. Most large river systems get secret gifts in this way, as they flow along, from thousands of springs that empty into them or their tributaries. So springs start and feed the rivers. Now what do you suppose starts the springs? Rain-drops stored away in big stone "safes," much as a small boy stores away pennies in his tin bank! The water of rains and melting snows, passing down through the soil, soaks into the little chambers or pores in such rocks as sandstone and limestone, and keeps going on down until it comes to a bed of hard stone, such as slate or granite, into which it cannot soak. [Illustration: THE SPRING WHEN EMPTY] [Illustration: THE SPRING WHEN FULL] THIS SPRING PLAYS IT'S A TOWN PUMP These two pictures show an intermittent spring about five miles from Singer Glenn, Virginia, and there called the "Tide Spring." You can see where the idea of the tide comes in, but can you think why the spring seems to have a tide system all its own? You know what a siphon is. Well, think how a kind of siphon might be formed in rock, dissolved out by water flowing underground. Then look at the picture on the next page. Now rock-beds, as you know, have a slope--some more, some less--owing to the wrinkling of the earth's crust. So the water, slowly trickling through the porous rock, forms a steady stream which runs down along the hard rock, as rain runs down a roof, and finally gushes out at some lower level. [Illustration: HOW THE LITTLE SPRING WORKS ITS PUMP This is how the pump of an intermittent spring is worked. Some portions of rock are dissolved by underground waters more readily than others and so cavities are sometimes formed, as shown. As long as the water in the reservoir is below the arch of the siphon-shaped outlet no water escapes, but as soon as it rises to the level of the arch the whole of the water is drawn off. Then the spring ceases to flow until the reservoir fills up again. You can empty water in the same way by using a bent tube of any kind. Can you tell why the water flows up-hill in this way? Remember what you know about air-pressure and then look up "siphon" in your encyclopædia. ]