ploughing, this protoxide frequently becomes exposed, and the consequence is that the fertility is impaired until the protoxide is converted into peroxide, and the red colour becomes apparent upon the surface. Struve has proved by experiments, that water which is impregnated with carbonic acid decomposes all rocks containing alkalies, and then dissolves a portion of the alkaline carbonates.

88. Soils being thus formed by the disintegration of rocks, their properties are evidently dependant, first, upon the nature of the several components of these rocks; and, secondly, upon the effects produced upon these components by the action of the air and water to which they are subsequently exposed. Of the various kinds of soil, the principal constituents are-1. sand, 2. lime, and 3. clay. The first two of these, containing no other inorganic substances except siliceous earth and carbonate or silicate of lime, afford no nutriment whatever for vegetation. The clay or argillaceous earth constitutes the fructifying element in all soils, and is produced by the disintegration of aluminous minerals, among which are the felspars, mica, &c. The fertilizing properties of argillaceous earths appears to arise from their containing alkalies and alkaline earths, with sulphates and phosphates, ingredients which are never absent from these earths. This valuable property of the argillaceous earths is also aided by the peculiarity of their texture, which affords great facilities for retaining moisture. Vegetable life, however, requires, besides the nourishing properties found in the argillaceous earths, to be supplied with air and moisture, and while alumina gives no aid to the passage of these essentials, chalk and sand do give it by their mechanical formation. Hence, "land of the greatest fertility contains argillaceous earth and other disintegrated minerals, with chalk

and sand in such a proportion as to give free access to air and moisture."* The clays are therefore to be regarded by the drainer as impermeable and retentive materials, and the sands and limes as porous materials; and the infinite proportions in which these matters are found combined in soils, determine the degree in which any soil will facilitate or impede the passage of water through it.

89. With this knowledge of soils, we may proceed to the arrangement of the drains required for regulating the supply of water to the lands of a district. This arrangement will be varied according to the contour of the surface, and the position of the substrata. If this be level, and the texture of the soil uniform, the drains may be at once planned, with mains at certain intervals, and minor drains or feeders at right angles to the mains, and parallel among themselves, as shown in fig. 32. The inclination in the bed of the drains, which

Fig. 32.

* Liebig.

is necessary to assist the discharge of their contents, must be obtained by cutting them deeper towards the receiving channel, as shown in fig. 33, which is a Fig. 33.

longitudinal section of one of the main drains, with the feeders discharging into it. This increase of depth also provides the additional capacity wanted in the drains as the water accumulates in them. An undulating surface will require the main drains to be arranged at the lowest levels, and conducting the minor channels into them with due reference to the capacity of the mains to discharge their united contents. Fig. 34 is a plan of a surface of this kind, the lines A B,

CD, and EF showing the position of the hollows in which the main drains are to be laid, the minor ones being arranged so as to divide the total discharge among the mains as nearly equally as the nature of the surface will conveniently admit. The capacity of the mains, as also of the feeders, must, of course, be determined according to the quantity of water for which passage is required, and modified also by the steepness of the fall. If the fall is considerable, a drain of smaller dimensions will suffice than will be necessary if but little inclination can be obtained. That part of the main drain from E to B will, in any case, require enlarged capacity, as it receives the entire drainage. Fig. 35 is a section of part of the mains, in which it will

Fig. 35.

be seen that, as the surface inclines, the bed of the drain will have sufficient fall if laid at equal depth from the surface of the ground throughout. The drains are arranged parallel to each other, which is evidently a good rule where it can be observed, the surface being thus divided into equal spaces, and the drainage made at once perfect and simple.

90. The arrangements of drains here described suppose the texture of the soil to be the same throughout the surface to be drained; but if soils of different retentive power appear upon the surface, it becomes essential to arrange the drains with reference to the line of junction of the soils. Thus, let figs. 36 and 37 represent the plan and section of a district, whereof the higher ground is of

porous materials, P overlying the clay, or other retentive soil, (marked R,) as far as the line A, D, B, where the clay first appears upon the surface. The water percolating through the bed P, and prevented from descending by the clay, will accumulate along the line A, D, B, and form a swamp unless got rid of. In this case, therefore, a drain must be laid along this line, while a main drain for the remainder of the ground, which slopes towards the brook at G, must be formed on the line E, Da, F. This latter drain receives the contents of the minor drains, which are to be laid