OF LAND. In reclaiming large traits of land, often of great fertility. but which are at so slight an elevation above the sea or the m•ighboring streams that they can be rendered tit for cultivation and habitation by artificial means only, dike or embankments are con -trot•led to keep out the flood waters and a system a f canals built to collect and carry off the superfluous water. In the ease of marsh land- the method employed depends upon the ele vation and el mformation of the land to be drained. lint all lands may be divided into two general that can be drained by gravitation and those in whirl] the water must be pumped out, either all or a part of the time. Many coast lands are so low that the water will run ,itr ,ady at how tide. and lands are pro tcela 41 from high tide by embankments. The water is gathered by a network of ditches into a few main drains, which discharge their contents into the sea at low tide, while the tidal flow is kept out by automatic doors or sluices. The problem of draining low lands by gravity is an increasingly ditlieult one, becau-c a 'section that is drained tends to sink. through shrinkage, below its former level. Hence it hap pens, as in East Lincolnshire, England, that land which at first was drained effectually by gravity afterwards. through subsidence, required pump ing to carry off the water. Much of the laud that has been most successfully reclaimed from encroaching water- ha- been so low that it re quired slumping to carry the drainage off during a whole or a part of the year. This is particu larly true in Holland. Many other considera tions, besides the actual level of the land to be drained. must be taken into account in devising a system of drainage for low' lands. The object sought in laying out drains in a flat country is to provide channels of the least possible area. depth, and inclination that will suffice to off the water properly. Every increase beyond this point is a waste of land and of expense in excava tion: and where pumping is necessary, the greater the inclination or grade of the canal, the higher will he the lift. Usually a much greater
area must lie devoted to ditches when the land is drained by gravity than by pumping, so that the increased expense for constructing and maintain ing ditches will sometimes equal the cost of in stalling and operating pumps.
In determining the number and capacity of drains required. the local rainfall and the ab sorbent power of the soil must be taken into ac count. Low areas. surrounded by higher land, often have not only the rainfall on their own area, hut also the drainage from the surrounding country to he provided for. This is prevented, whenever practicable. by separating the low land from the surrounding country by a eateh-drain !q.v.). Care should he taken that the subsidiary ditches or drain-pipes do not discharge into the main canal at a point below- the water-level of the latter, for if this happens. the free circulation of air through the soil by means of the drains is impeded and there is danger that the water will back up. Drainage canals should he kept free from weeds. for their growth impedes the flow of water, by decreasing the area of the ditch and by increasing the friction of the running water. Various machines have been devised for cutting the growth of weeds in ditches and for digging them out by the roots. Having eonsidered some of the most general principles underlying the con struetion of a drainage system, a few of the most important examples of land reclamation will he deseribed.
hue of the best-known and most extensive sys tems of land reclamation is found in 'Holland. Here the greater part of the country is low and flat. and is protected from intimidation by an enormous system of dikes. The soil was origi nally marshy and the territory included nume rous lakes. By an elaborate and scientific system of canals many of these marshes and lakes. now called polder-bind, have been drained and Con verted into fertile soil. ( Sep MILDER.) most famous of these polders is the former site of the Haarlem Lake, where a population of nearly 20.000 people are now dwelling. Another of these polders is the Biesboseh.