The third branch in the art of draining is by far the most expensive operation, in conse quence of the number of drains required to lay the surface dry, and the necessity of filling them with porous substances, through which the surface water can penetrate. There is often a layer of light earth immediately over a substratum of clay, and after continued rains this soil becomes filled with water, like a sponge, and no healthy vegetation can take place. In this case numerous drains must be made in the subsoil, and over the draining tiles or bushes which may be laid at the bot tom of the drains loose gravel or broken stones must be laid to within a foot of the surface, so that the plough shall not reach them. The water will gradually sink into these drains and be carried off, and the loose wet soil will become firm and dry. The slope of the field and the fall which can be obtained for the drains, their size and depth below the surface. the angles at which they intersect each other, and their number—all are circumstances which require the drainer's best attention. In drain ing clay land, where there is only a layer of a few inches of loose soil over a solid clay which the plough never stirs, the drains need not be deeper than two feet in the solid clay, not wider than they can be made without the sides falling in. The common draining tile, which is a fiat tile bent in the form of half a cylinder, is the best for extensive surface draining. In solid clay it requires no fiat tile under it ;.it is merely an arch to carry the loose stones or earth with which the drain is filled up. In grass land the sod may be laid over the drain after it has been filled up, so as to form a slight ridge over it. Sometimes a drain is covered with a sod, without any tile whatever ; and at ether times a twisted rope of straw is thrust into the drain. Draining tiles are now made of various shapes and sizes.
As the draining of wet clay soils is the only means by which they can be rendered profit able as arable land, and the expense is great, various instruments and ploughs have been contrived to diminish manual labour and ex pedite the work. Among these are the com mon mole plough, the draining-plough, Smith's subsoil plough, tapering spades, and hollow spades. Weir's improved draining level is one of the instruments now often employed in these works.
The draining of Haarlem Lake is one of the greatest hydraulic enterprises ever con ducted. We have nothing in this country that can give an adequate idea of this lake, or the necessity of its being drained. Three centu ries ago there were several small lakes in the flat district which lies between the towns of Amsterdam, Leyden, and Haarlem, in Holland; but by the gradual wearing away of the soft ground between them, many of these became converted into one large lake ; and the shores continued to be washed away so extensively, that by the beginning of the 18th century a lake was formed covering an area of 45,000 acres. As the level was 13 feet below the level of the neighbouring sea, there was no outlet for the water ; and although the Dutch incurred heavy expenses to endeavour to arrest the enlargement of the lake, they had no means of diminishing it. At length, in 1836,
two inundations occurred of so serious e, cha racter, as to determine the Dutch government to drain the lake at any cost ; one inundation covered 10,000 acres of low land near Amster dam, while another covered 19,000 acres near Leyden. The first work was to dig a canal entirely around the lake, 38 miles long, 120 feet wide, and 9 feet deep ; this canal was to accommodate the immense water traffic which was conducted on the lake, and to serve as a receptacle for the water of the lake. The next point was, to close with large earthen darns all inlets into the lake, so that it should have no increase, except from rain water. Other hydraulic works were executed in other places, to facilitate the flow of water into the sea as soon as it was raised from the lake. These preliminary works occupied the time from the year 1840, when the enterprise was determined on, till 1845, when the steam engines were set to work. These engines are the largest that have ever been constructed; they were made by Cornish engineers, who have acquired great skill in the manufacture of steam-engines which shall perform a great amount of work with a small amount of fuel. Each steam engine has an enormous cylinder 12 feet in diameter, with an annular or ring shaped piston ; and within this piston cylinder is another cylinder and piston 7 feet in di ameter. Eleven pumps, more than 5 feet in diameter of barrel, are ranged around a sort of tower, on which the steam engine is placed; and all these enormous pumps are worked at once by the steam engine. The water is lifted by the pumps from the lake into the canal, whence it flows by sluices into the sea. It was calculated that about a thousand million tons of water would have to be lifted from the lake; and that after it was drained, there would regularly be about fifty millions of tons that would require to be annually removed, the result of drainage and rain. It will servo to convey some idea of the enormous power of the engines employed, that each engine raises 112 tons of water 10 feet high at one stroke.
A draining operation of great magnitude is about to be commenced at the astuary called the Wash. This sestuary forms a sort of bay between Norfolk and Lincolnshire, and into it flow the rivers Witham, Nene, Welland, and Ouse. It has become so silted up, that a large area is utterly useless—too shallow to be available for navigation, and too wet to be used for agricultural purposes. To deepen the mouths of all the rivers, and to reclaim many thousand acres of marshy useless land, are the great objects of the Wash Drainage enterprise. All the rivers will be greatly im proved by these works ; while it is estimated that the reclaimed land, as an agricultural estate or estates, will pay for the operation, and yield an adequate profit. The towns o Lynn, Wisbeach, Spalding, and Boston, situ ated respectively on the four rivers, will all be benefited by the operation, as vessels of larger tonnage will be enabled to reach those towns than can ascend the present partially-choked streams.