Fig. 12. Sand from Penhale sand dunes blowing on to and covering up meadows
Sand particles, being large, do not float in water. If we shake up sand in water the sand sinks, leaving the water entirely clear. So running water does not carry sand with it unless it is running very quickly: the sand lies at the bottom.
Unlike clay, sand does not hold water. Pour some water on to sand placed on the tin disk in a funnel (Fig. 8); it nearly all runs through at once. We should therefore expect a sandy field or a sandy road to dry up very quickly after rain and not to remain wet like a clay field. So much is this the case that people prefer to live on a sandy soil rather than on a clay. The most desirable residential districts round London, Hampstead on the north, and the stretch running from Haslemere on the south-west to Maidstone on the south-east, and other favoured regions, are all high up on the sand.
At the foot of a hill formed of sand you often find a spring, especially if clay or solid rock lies below. It is easy to make a model that will show why the spring forms at this particular place. Fill the lower part of the box shown in Fig. 13 with wet clay, smoothing it out so that it touches all three sides and the glass front; then on top of the clay put enough sand to fill the box. Bore four holes in the side as shown in the picture, one at the bottom, one at the top, one just above the junction of the sand and clay, the fourth half way up the sand, and fix in glass tubes with clay or putty. Pour water on to the sand out of a watering can fitted with the rose so as to imitate the rain. At first nothing seems to happen, but if you look closely you will notice that the water soaks through and does not lie on the surface; it runs right down to the clay; then it comes out at the tube there (c in the picture). None goes through the clay, nor does enough stay in the sand to flow out through either the top or the second tube; of the four tubes only one is discharging any water. The discharge does not stop when the supply of water stops. The rain need only fall at intervals, but the water will flow all the time.
Fig. 13. Model spring. A box with glass front contains a layer of clay and one of sand. Water that falls on the sand runs right down to the clay but can get no further, and therefore flows out through the tube c at the junction of the clay and the sand. The same result is obtained when chalk takes the place of sand
The experiment should now be tried with some chalk from a quarry; it gives the same results and shows that chalk, like sand, allows water readily to pass.
