Fig. 15. "The little pool below the tree"
Now stop up the glass tubes so that the water cannot get out. Soon the sand becomes flooded and is no better than clay would be. A second model will show this very well. Make a large saucer of clay and fill with sand: pour water on. The water stays in the sand, because it cannot pass through the clay. A sandy field saturated like this will therefore not be dry, but wet, and will not make a good position for a house. We must therefore distinguish the two cases illustrated in Fig. 17. A shows sand on a hill, dry because the water runs through until it comes to clay or rock, when it stops and breaks out as a spring, a tiny stream, or pond; this is a good building site and you may expect to find large houses there. B shows the sand in a basin of clay, where the water cannot get away: here the cellars and downstairs rooms are liable to be wet, and in a village the wells give impure water. Matters could be improved if a way out were cut for the water, but then the foundations of the buildings might move a little.
