r
foundations shrinking away. {11} We can now understand why some of our
model bricks cracked. The cracks were caused by the shrinkage just as
happens with our model field. As soon as the clay becomes wet it
swells again. A very pretty experiment can be made to show this. Fill
a glass tube or an egg-cup with dry powdered clay, scrape the surface
level with a ruler, and then stand in a glass jar full of rain water so
that the whole is completely covered. After a short time the clay
begins to swell and forces its way out of the egg-cup as shown in Fig.
5, falling over the side and making quite a little shower. In exactly
the same way the ground swells after heavy rain and rises a little,
then it falls again and cracks when it becomes dry. Darwin records
some careful measurements in a book called _Earthworms and Vegetable
Mould_--"a large flat stone laid on the surface of a field sank 3.33
millimetres[1] whilst the weather was dry between May 9th and June
13th, {12} and rose 1.91 millimetres between September 7th and 19th of
the same year, much rain having fallen during the latter part of this
time. During frosts and thaws the movements were twice as great."

[Illustration: Fig. 4. Clay was plastered over a square piece of board
and completely covered it. After drying for a week the clay had shrunk
and cracked]

You must have found out by now how very slippery clay becomes as soon
as it is wet enough. It is not easy to walk over a clay field in wet
weather, and if the clay forms part of the slope of a hill it may be so
slippery that it becomes dangerous. Sometimes after very heavy rains
soil resting on clay on the side of a hill has begun to slide downwards
and moves some distance before it stops. Fortunately these land slips
as they are called, are not common in England, but they do occur. Fig.
6 shows one in the Isle of Wight, and another is described by Gilbert
White in _The Natural History of Selborne_.

[Illustration: Fig. 5. Clay swelling up when placed in water and
overflowing from the egg-cup into which it was put]

[Illustration: Fig. 6. Landslip in the Isle of Wight]

Another thing that you will have noticed is that anything made of clay
holds water. A simple way of testing this is to put a round piece of
tin perforated {14} with holes into a funnel, press some clay on to it
and on to the sides of the funnel (Fig. 7), and then pour on rain
water. The water does not run through. Pools of