THE SPRING WHEN EMPTY
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.
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.