mains are also required at D D, to drain the clay surface above these points, and save the porous bed from the saturation which will naturally occur unless thus prevented.

104. In hilly districts, clays and gravel are often found in alternate layers, which outcrop on one side of the hill, as sketched in fig. 50, and render a series of

main drains necessary at the points marked D. By these drains, the water which gathers in the retentive strata will be discharged at the lowest points on the surface, and prevent any mischievous excess on the soil. The intermediate portions of the porous materials which

are exposed will readily get rid of their contents by percolation, and drains of comparatively small dimensions will be adequate to the efficient drainage of a section thus composed.

105. Sometimes the side of a hill displays a series of alternate and horizontal layers, as represented in fig. 51,

in which case a small main drain should be laid at the exposed bed of each stratum, at the points marked D D, which will receive the contents of each porous layer, and prevent any injurious excess accumulating within the intermediate clays.

106. Having thus briefly noticed the several varieties of section which are likely to occur in the drainage of districts and lands, we have now to consider the form, size, and construction of drains which it will be advisable to adopt according to the circumstances of each

case.

107. The rudest form of drain is that of an open cut or channel in the surface of the ground, for conveying the water which falls in the form of rain, or percolates through the materials intersected, away into some lower

position, brook, or other receiver. These open drains are distinguished from the more complete form of underground or covered drains formed by open channels, which are afterwards refilled, except at the lower part, along which a channel is preserved by one of several methods of construction. Both of these methods appear to be of great antiquity, having been certainly practised by the Romans, as recorded by Palladius, Pliny, &c. Open drains are applicable only as conductors of surface water, and for strong tenacious soils. To make them effective in draining from the body of the soil, the depth necessary renders open drains inadvisable; while, in loose soils, the inclination of the sides, which must be allowed in order to prevent their rapid destruction, occupies a most extravagant surface of the land. They are evidently inapplicable to land submitted to the plough, by which they are almost certain to be injured or destroyed, and thus have commonly been restricted. to pasture land, whence they have been named sheepdrains. Even as thus limited, the use of open drains is of very doubtful advisability, inasmuch as they are always much exposed to injury, and to have their banks trodden down and destroyed. Admitting permanent utility as an object in drain-making, it is certain that covered drains should, in nearly all cases, both for arable and pasture districts, be preferred to open ones.

108. Covered drains, being simply intersticial courses formed beneath the surface, may be constructed in a great variety of ways, which may be partly determined by the proximity of the suitable materials. One of the simplest forms, and most generally applicable, consists of a layer of stones in the bed of the drain, which is afterwards filled up with the soil taken out of it in order

to deposit the stones, as shown in fig. 52. In these drains there is a liability to become less active, by particles of soil being forced down or brought into the water, and clogging the spaces left for its passage. If stratified stone is cheaply obtainable, the better arrangement represented in fig. 53 should be adopted, consisting of side stones, and one cover over them, leaving an open space or duct through which the drainage water passes of course more fluently than through the spaces between the stones, as shown in fig. 52. A compound drain, composed of a layer of loose stones, and an artificial duct formed with a flat tile on the bed of the drain, and covered with a semi-cylindrical tile, as shown in fig. 54, combines the advantages of the two preceding drains. This form is commonly denominated the sole and tile drain, and may, in most parts of the country, be constructed at less cost than the stone duct shown in fig. 53. It has also the advantage of greater permanency, being less liable to displacement of the parts. In the drain shown in fig. 53, the same arrangement of stones over the duct may of course be introduced; but unless the work is very carefully done, and the covering with the flat stones rendered perfect, the loose stones are liable to fall into the duct, and thus

destroy its utility. In clays and tenacious soils, drains such as that shown in fig. 55 are sometimes formed by

cutting the lower part narrower on each side, and thus leaving shoulders, on which a flat stone being supported, an open space is left below, forming a natural duct or open passage for the water. The permanence of this, the shoulder drain, is somewhat insecure, as it depends solely upon the shoulders being preserved, and the qualification of the material to resist all damage to the open parts of the drain. Another form of rough stone drain is represented in fig. 56, for which the larger stones are assorted, and placed in the bed with a layer of small stones upon them. It must be remarked of this, however, as of every rough stone drain, that its permanent action, depending upon the small spaces left between the stones, is very liable, in the course of time, to become much impaired or destroyed by the particles of soil and solid matters brought along with the drainage water; and on this account, especially, these drains are far inferior to those constructed with permanent open ducts. Fig. 57 shows a drain suitable for bog and peaty soils, with which the drain is filled up, leaving an open space below for the passage of the water. The principal objection to this form of construction in peat is, that the