Sometimes land may be drained by boring through the clay or other impervious soil, down to a porous stratum, such as sand or gravel. A knowledge of the structure of the whole district is necessary to determine whether this is likely to be of service or not. By boring deep pits in the drains cut in peat mosses, the weight of the mosses will cause the water to be squeezed out, which rising in these holes flows off by the drain, producing the same effect as if it were cut to the depth of the pits. Drains are frequently made by means of narrow curved spades, but the draining plough effects the purpose more rapidly, and is very generally used.
The drainage of London and other large towns consists of small drains from each house, made of earthenware tubes leading to a sewer of brickwork, running along each street and uniting with larger ones. The question of effectually draining London is a very difficult one, schemes of various kinds being at this time propounded, which would cost several millions of pounds to put into practice; at present all the sewage runs into the river Thames, and it is a matter of serious importance to obviate this, by carrying it off to some other place.
Illustration: FIG. 2. (‡ TAPPED BENT TUBE.)
Illustration: FIG. 3. (‡ ARTESIAN BASIN.)
The construction of artesian wells depends upon the fact that water, being at liberty to flow, will always sink to a level; by which it is meant that the parts which are highest press upon those that are lower, and tend to raise them, the higher parts sinking in the same proportion that the lower parts are elevated; this continues until both are upon a level. Suppose a bent glass tube of the form of fig. 1, be partly filled with water, the surfaces of the water in both arms of the tube will each be upon a level with the other; suppose now another such tube to have a small hole at A, the water sinking in each arm will force out a jet of water, and if a tube be inserted into this hole it will represent an artesian well, and the water will rise in it till all three are on a level (fig. 2). Instead of these tubes there is a layer of some porous material, as gravel, at some distance beneath the surface of the earth, rising at each end and forming a sort of basin (fig. 3), which is bounded above and below by some impervious substance as clay or stone; the well being sunk at any part (as at _a_, _b_, or _c_) below the level of the gravel, where it forms the surface of the earth, must cut through the upper stratum of clay or stone, and thus form a tube into the porous gravel which holds the water; this water is obtained from the rain, which, falling on the surface of the earth, drains through the gravel and fills its lower part. The water will rise in the bore at a height according to circumstances, if the gravel at each side of the bore rises to higher ground, a jet of water will be forced out, if not so high, the well will only partly fill, and so on. Artesian wells are sunk or bored by means of instruments screwing on to the end of a set of iron rods each screwing into the end of the other as shown in fig. 4. The first piece is generally a sort of gouge (_a_) two or three inches wide, and so made as to bring up a cylindrical piece of earth when forced in and screwed round, the weight of the rods after a few pieces are added is sufficient to force the gouge into the earth, except in stony places, when a sort of “pick” is used.