icture, 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 {26} 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.

[Illustration: 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.

[Illustration: Fig. 14. Foot of a chalk hill at Harpenden where a
spring breaks out just under the bush at the right-hand side of the
gate]

Just the same thing happens out of doors in a sandy or chalky country;
the rain water soaks through the sand or chalk until it comes to clay
or solid rock that it cannot pass, then it stops. If it can find a way
out it {28} does so and makes a spring, or sometimes a whole line of
springs or wet ground. Rushes, which flourish in such wet places, will
often be found growing along this line, and may, indeed, in summer time
be all you can see, the water having drained away. But after much rain
the line again becomes very wet. Fig. 14 shows the foot of a chalk
hill near Harpenden, where a spring breaks out just under the bush at
the right-hand side of the gate. In Fig. 15 the bush itself is seen,
with the little pool of water made by the spring. Here the water flows
gently, but elsewhere it sometimes happens, as in Fig. 16, that the
spring breaks out with great force.

[Illustration: Fig. 15. "The little pool below the tree"]

Now stop up the glass tubes so that the water cannot get out. Soon th