DRAINAGE. 429 DRAINAGE. ditcOics into a few main drains, which discharge their contents into the sea at low tide, while the tidal How is kept out by automatic doors or sluices. The problem of draining low lands by gravity is an increasingly dillicult one, because a section that is drained tends to sink, tlirough shrinkage, below its Kuiiier level. Hence it hap- pens, as in East Lincolnshire, Kngland, that land which at first was drained eHeclually by gravity afterwards, through subsidence, required pump- ing to carry off the water. Much of the land that has l)een most successfully reclaimed from encroaching waters has been so low that it re- quired pumping 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 carry off the water properly. Every increase beyond this point is a waste of land and of expense in excava- tion : and where pmnping is necessary, the greater the inclination or grade of the canal, the higher will be the lift. Usually a much greater area must be 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, but also the drainage from the surrounding country to be provided for. This is prevented, whenever practicable, by separating the low land from the surrounding country by a catch-drain ( q.v. ) . Care should be 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 wat^r will back up. Drainage canals should be 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 considered some of the most general principles underlying the eon- •^truction of a drainage system, a few of the most important examples of land reclamation will be described. One 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 inundation by an enormous system f)f dikes. The soil was origi- nally marshy and the territorv included nume- rous lakes. By an elaborate and scientific system of canals many of these marshes and lakes, now called finldrr-lnnd, have been drained and con- verted info fertile soil. ( See Por.OKR. ) Tlie most famous of these polders is the former site of the Haarlem Lake, where a population of nearly 20.000 people are now dwelling, .• other of these polders is the Biesbosch. The Haarlem Meer was formed in the six- teenth and scventcj-nth centuries by the junction of several small lakes. Three villages were cov- ered with water, the last in 1647. and much otherwise valuable territory was rendered useless. Ot the various early plans for relief, one pro- posed by Leeghwater, in lti43, contemplated an embankment around the whole area, and IGO windniill-drivcn pumps, with an estimated cost of about if 1,500,000. In IS.'ifi the Meer was extended still further, and in 1837 work was Iwgun for the reclamation of 44,724 acres of land, covered with some thirteen feet of water. An embankment 371^ miles long was built around the lake, and a canal then in existence was improved and extended, partly for naviga- tion, it was decided to use steam-power for pumping, and a pumping-engine was adopted of the single-acting type. It had a high-pressure cylinder inside the low-pressure one. and a cross-head coupled to eleven beams; on the outer end of each was a single-acting pump. These pumps were arranged in a circle. The first en- gine had a high-pressure cylinder of S4V4 inches diameter: low-pressure, "14414 inches. The pump cylinders were 63 inches in diameter. The stroke of both the steam and pump cylinders was 10 feet. The beams were 32 feet 10 inches long. Each of the five boilers was 6 feet in diameter, and 30 feet long, with a single 4-foot flue. There were three engines in all. the last two to be erected being similar to the one de- scribed, but having a 73-inch pump cylinder. The first engine lowered the lake si.x inches in about ten months, after which it took the three engines 39 months to complete the work. For the six years ending about 1S44, the first, or Leyden engine, ran at an average speed of 514 strokes per minute, discharging 238 long tons of water per minute. At the close of that period this pumping-engine was replaced by a centrifugal pump, having a 58-inch inlet, and driven by an inverted direct-acting compound engine, designed to run at 100 revolutions per minute, and to give 550 indicated horse-power. On a trial, this plant, with the engine running at 90.9 revolutions per minute, and with a lift of 14.97 feet, delivered 293 long tons of water per minute for some hours: the maximum for one hour being 342 tons, lifted 15.34 feet. With the exception of Holland, the Fenland of England is a region where the science of drainage has probably been applied to the improvement of land on a larger scale than in any other part of the world. The district known as The Fenss is a tract of fiat and marshy land on the east coast of England, having an area of about 400.000 acres. The whole region was, centuries ago. converted into an tniprofit- able marsh by repeated incursions of the sea, coupled with obstructions to the outward flow of the rivers Xene. Cam, Ouse. Wclland. etc. Vast operations have been carried on ever since the time of Charles I., by digging new channels and outfalls, and employing windmills and steam-engines to pump the water from the marshes and ponds into these artificial channels. The region is divided into the Xorth. the Middle, and the l?outh Levels, managed by commission- ers, who^e powers are derived from special acts of Parliament. The improved value of the land is the fund out of which the expense of the engi- neering works is defrayed. It was in one of these districts (the Middle Level, between the
