SOME EXPERIMENTS WITH THE SAND
Apparatus required.
_Sand, about 6 lbs.; clay, about 6 lbs. Six funnels, stands and disks
[1]. Six glass jars [2]. One box with glass front shown in Fig. 13
filled with clay and sand, as indicated. Quarry chalk (about 5 lbs.).
Six beakers [1]. Six egg-cups [1]._
If there is a sand pit near you, or a field of sandy soil, you should
get a supply for these experiments; if not, some builder's sand can be
used. When the sand is dry you will see that the grains are large and
hard. Further, they are all separate 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 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 {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 the
sand becomes flooded and is no better than clay would be. A second
model will show this very well. Make a large saucer of clay and fill
with sand: {30} pour water on. The water stays in the sand, because it
cannot pass through the clay. A sandy field saturated like this will
therefore not be dry, but wet, and will not make a good position for a
house. We must therefore distinguish the two cases illustrated in Fig.
17. _A_ shows sand on a hill, dry because the water runs through until
it comes to clay or rock, when it stops and breaks out as a spring, a
tiny stream, or pond; this is a good building site and you may expect
to find large houses there. _B_ shows the sand in a basin of clay,
where the water cannot get away: here the cellars and downstairs rooms
are liable to be wet, and in a village the wells give impure water.
Matters could be improved if a way out were cut for the water, but then
the foundations of the buildings might move a little.
[Illustration: Fig. 16. Water bursting out from an underground spring,
Old Cateriag Quarry, Dunbar]
It often happens that villages are situated at the junction of sand and
clay, or chalk and clay, because the springs furnish forth a good water
supply.
On the other hand large tracts of clay which remain wet and sticky
during a good part of the year are not very attractive to live in, and
even near London they were the last to be populated: Hither Green in
the south-cast and the clay districts of the north-west have only of
late years been built on; while the sands and gravels of Highgate,
Chiswick, Brentford and other places had long been occupied.
Elsewhere, villages on the clay do not grow quickly unless there is
much manufacturing or mining; the parishes are large, the roads even
now are not good while they used to be very bad indeed. Macaulay tells
us that at the end of the seventeenth century in some parts of Kent and
Sussex "none but the strongest horses could in winter get through the
bog, in which at every step they sank {31} deep. The markets were
often inaccessible during several months. . . The wheeled carriages
were, in this district, generally pulled by oxen. When Prince George
of Denmark visited the stately mansion of Petworth in wet weather, he
was six hours in going nine miles; and it was necessary that a body of
sturdy hinds should be on each side of his coach to prop it up. Of the
carriages which conveyed his retinue several were upset and injured. A
letter from one of the party has been preserved in which the
unfortunate courier complains that, during fourteen hours, he never
once alighted, except when his coach was overturned or stuck fast in
the mud." The Romans knew how to make roads anywhere, and so they made
them run in a straight line between the two places they wished to
connect, but the art was lost in later years, and the country roads
made in England since their time usually had to follow the sand or the
chalk, avoiding the clay as much as possible. These roads we still
use. Fig. 18 shows the roads round Wye; you should in your rambles
study your own roads and see what soil they are on.
[Illustration: 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 {32} 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).
[Illustration: 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)]
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