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 of leaves in autumn and let them "rot down" and change into mould. If you can in autumn collect enough leaves to make a heap you {34} should do so and leave it somewhere where the rain can fall on it, but cover it with a few small branches of trees to prevent the wind blowing the leaves away. The heap shrinks a great deal during the first few months, and in the end it gives a supply of mould that will be very useful if you want to grow any plants in pots. Some of the little hollows in the bank under a hedge, especially on chalky soils, are filled with leaf mould which has sometimes changed to a black powder not looking at all like leaves. You can also find mould in holes in decayed trees; here it has formed from the wood of the tree.