Fig. 17. Two positions of sand. A is dry because the water can drain away and break out as a spring at c. B is wet because the water cannot drain away
There are several other ways in which sand differs from clay. It does not shrink on drying nor does it swell on wetting, and you will find nothing happens when you try with sand the experiment with the model field (p. 11) or the egg-cup (p. 12).
Fig. 18. The roads round Wye. As far as possible they keep off the clay (the plain part of the map) and keep on the chalk or the sand (the dotted part of the map)
CHAPTER V
THE PART THAT BURNS AWAY
Apparatus required.