"In drying under the influence of the sun," says Prof. Johnston, "soils shrink in, and thus diminish in bulk, in proportion to the quantity of clay, or of peaty matter, they contain. Sand scarcely diminishes at all in bulk by drying; but peat shrinks one-fifth in bulk, and strong agricultural clay nearly as much." By laying drains in land, we take from it that portion of the water that will run out at the bottom. The sun, by evaporation, then takes out a portion at the top. The soil is thus contracted, and, as the ends of the field cannot approach each other, both soil and subsoil are torn apart, and divided by a network of cracks and fissures. Every one who is familiar with clay land, or who has observed the bottom of a ditch or frog pond by the roadside, must have observed these cracks, thus caused by the contraction of the soil in drying. The same contraction occurs in drier land, by cold, in Winter; by which, in cold regions, deep rents are made in the earth, and reports, like those of cannon, are often heard. The cracking by drying, however, is more quiet in its effects, merely dividing the ground, noiselessly, into smaller and smaller masses, as the process proceeds. Were it not for this process, it may well be doubted whether clay lands could be effectually drained at all. Nature, however, seems to second our efforts here, for we have seen that the stiffer the clay, the greater the contraction, and the more the soil is split up and rendered permeable by this operation. These cracks are found, by observation, to commence at the drains, and extend further and further, in almost straight lines, into the subsoil, forming so many minor drains, or feeders, all leading to the tiles. These main fissures have numerous smaller ones diverging from them, so that the whole mass is divided and subdivided into the most minute portions. The main fissures gradually enlarge, as the dryness increases, and, at the same time, lengthen out; so that, in a very dry season, they may be traced the whole way between the drains. The following cut will give some idea of these cracks, or fissures, as they exist in a dry time: [Illustration: Fig. 98.--Cracking of Clays by Drainage.]