At the present time, it is said, the Dutch government has under con sideration the drainage of the Znyder Zee—a project of even greater mag nitude and importance than that of the Haarlem Meer. The preliminary studies have been carefully made under the direction of the government engineers, who have pronounced the project to be technically and econom ically feasible. The area to be drained, and thus recovered for the pur poses of agriculture, embraces nearly four hundred thousand acres, and is covered with water to an average depth of 13 feet. It is estimated that to effect this colossal work ninety-four hundred horse-power, divided between sixty-three steam pumping-engines of one hundred and fifty horse-power each, will be required. The time for the complete drainage of the area is placed at twenty-one months, and the cost is estimated at 116,000,000 Dutch guldens ($46,0oo,000).
Sezecrage.—Aniong drainage works must be included the covered canals or sewers under the streets of cities. These are intended to carry off underground and to conduct to the river or the sea the rain-water and the sewage, which otherwise would have to run off in the open gutters. There is some difference of opinion among engineers as to the propriety of using two systems of channels, or of discharging both rain-water and sew age through the same channels. The first is known as the " separate " system; the latter, as the " combined " system. Both have their advo cates. In the separate system in operation in the city of Paris, the rain water and all the ordinary waste-water of the houses are led into the sew ers, but the excreta from four-fifths of the population and the solid portions from the remaining fifth are excluded, and are removed in closed casks or retained in cesspools, from which they are pumped by odorless excavators. The municipal engineers of Paris, however, are said to favor complete water-carriage, and the changes that are constantly being made have in view the ultimate adoption of this system. In Germany opinions of muni cipal engineers differ as to whether it is better to remove the excreta by means of casks (usually' termed " dry removal ") or through tlie sewers (termed " water-carriage "), and both systems are in vogue.
Liernur System of Sezveragc.—In tlie so-called " Liernur" system the separate plan is developed to the highest state of perfection. In this, the excreta and a certain portion of the house waste-water are led into a system of iron pipes and removed pneumatically by means of an engine at a cen tral station. The remaining house-water, together with the rain-water, is carried off through a separate system of brick sewers.
Each of the plans, as indicated above, has its advocates, and the ques tion of deciding between them should be determined by an examination of the conditions prevailing in each locality.
Construction of Sewers.—The fall of the sewers should be as great as
possible, though the nature of the ground often compels the engineer to lay them almost horizontally. That they may be kept constantly free and as clean as possible, they should be so constructed that they can be entered and inspected. The bottoms should be smooth, solid, and impervious, and as far as practicable the entire channel should be provided with smooth interior surfaces. Fig,ure zo Ce5/. 57) is a view of a sewer with an oval section, showing the mode of connecting the street-gutters. Figure to shows a sewer of circular section, over which is placed a subterranean foot way; Figure 21 shows a section of one of the main collecting-sewers of Paris, with lateral footways for convenience of inspection. Of the two smaller cement-pipes seen at the sides, one is designed for the under-drain age of the sandy soil of the city, and the other for the future application of a system of universal underground drainage of excreta.
The Cleansing of Me Sewers is effected mainly by flushing, there being stored for the purpose a large body of water, which at stated intervals is suddenly let into the sewer, and which washes out the deposited sediment. This flushing process is sometimes accomplished naturally by the tide where the mouths of the sewers terminate in waters subject to ebb and flow. At convenient distances apart, the sewers are provided with man-holes or entrance-shafts to give admission to those charged with the work of inspect ing, repairing-, and cleansing them, and in most cases, also, air-shafts are provided, to insure ventilation.
Colusalage.—The drying of soils by elevation of the ground-surface may frequently be applicable in situations where the aboye-described plans for drainage are found to be inadmissible. This method has incidentally the advantage of permitting the soil to be improved at the same time, so as to fit it at once for cultivation. There are several ways in which this process may be carried on. In one of these the filling material is trans ported from other localities by men or by draught-animals, but this method, being comparatively costly, is confined to surfaces of limited area. In the other—which is more generally followed, and is applicable to surfaces of considerable extent—the filling is effected by artificially flooding- the low land with the silt-laden waters of adjacent streams. By means of embank ments and ditches judiciously disposed, the muddy flood-waters of such streams are conducted to and distributed over the surface upon which it is desired they shall deposit their solid matter. This process—termed by Continental Cligineers " colusatage"—has occasionally been successfully applied on aii extensive scale ; as, for example, in the Chiana River Val ley, in Central Italy, and the marshes of Grossotto, in Northern Italy.