and do not stick together; if you
make a hole in a heap of the sand, the sides fall in, there is nothing
solid about it, and you can easily see the mistake of the foolish man
who built his house upon the sand. When the sand is wet it sticks
better and can be made into a good many things; at the seaside you can
make a really fine castle with wet sand. But as soon as the sand dries
it again becomes loose and begins to fall to pieces.

[Illustration: Fig. 11. Sand dunes, Penhale sands, Cornwall]

Strong winds will blow these fragments of dry sand about and pile them
up into the sand hills or dunes common in many seaside districts (Fig.
11). Blowing sands can also be found in inland districts; in the
northern part of Surrey, in parts of Norfolk and many {24} other places
are fields where so much of the soil is blown away by strong winds that
the crops may suffer injury. In Central Asia sand storms do very much
harm and have in the course of years buried entire cities. Fig. 12
shows the Penhale sands in Cornwall gradually covering up some meadows
and ruining them.

[Illustration: 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 p