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)]

{33}

CHAPTER V

THE PART THAT BURNS AWAY

Apparatus required.

_Leaf mould. Mould from a tree. Peat. About 1 lb. soil from a wood,
a well-manured garden and a field; also some subsoil. Six crucibles or
tin lids. Six tripods, pipe-clay triangles, and bunsen burners or
spirit lamps. Six beakers and egg-cups [1]._

In the autumn leaves fall off the trees and form a thick layer in the
woods. They do not last very long; if they did they would in a few
years almost bury the wood. You can, in the springtime or early summer
find out what has happened to them if you go into a wood or carefully
search under a big hedge in a lane where the leaves were not swept
away. Here and there you come across skeleton leaves where only the
veins are left, all the rest having disappeared. But generally where
the leaves have kept moist they have changed to a dark brown mass which
still shows some of the structure of a leaf. This is called leaf
mould. The top layer of soil in the wood is soft, dark in colour, and
is evidently leaf mould mixed with sand or soil.

Leaf mould is highly prized by gardeners, indeed gardeners will often
make a big heap