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._------_--.TIIC, DUTCH POLDER..…


TIIC, DUTCH POLDER.. and AQUEDUCTS. The largest and richest portion of the soil of Holland lies, at high water, below the level of the sea and bein4 thus exposed to the two extremes of ebb and flood, which vary every six hours, it would of course lie under water, and be left high and dry, every six hours alternately, were it not for the bulwarks which the downs or sandhills afford along its coasts, aided its the work of conservation is by the dams and dikes with which the Dutch have guarded the banks of their rivers »nd lakes. Nay, in despite of all these barriers, Holland would be irre- coverably lost, and revert to her original condition of a swamp, if she had no means of ridding herself ot the im- mense quantity of water, which fs daily accumulating within her bounds by the effect of thaws, fogs, rains, springs, apd the natural porosity of her soil. Nature seems to have suggested to the Dutch, that the simpiiest and readiest mode of overcoming thisevit was to avail theinseiseti of the phenomenon of the flux and reflux of the tide. This is the leading principle, on which the greater part of their hydrotechnical works are constructed. Mention has been constantly made of late, both in the Dutch and Belgian reports, 6f the Poldei-s these are meadows in very low situations, which require :hat the water should be artificially expelled; and this is accomplished by means of sluices, which open out towards the sea or into the larger class of rivers, so soon as the pressure of the water behind is enabled to act upon them, under the effect of the ebb-tide, which removes the counterpressurc occasioned by its flax. The latter, on the other hand, closes the sluice-gates by its advance. and prevent the ingress of any water from without; whilst the internal water is afforded time to collect in the various dr .ins and ditches, which clear the land through these sluices at the time of ebbing. There is no inconsiderable portion of the Dutch Polders also, which lie much below low- water mark they were originally the site of moors. swamps, lakes, &c., which have been converted by ar- tificial means into dry land and the water which collects in them, is got rid of by the aid of windmills, which dis- charge it into canals; the latter disemboguing themselves of the superabundant fluid at ebb-time. All the water channels existing in the maritime provinces which are bounded by the Maas, Leek, and Y. form a series of hydro-tcchnic systems, centering in the lake of H aarlem this is a lake formerly in direct connexion with the Y and Sueder Sea, and therefore subject to the influence of tha tides but that influence has been arrested by the great sluices of Sparepdam and Halffweg, which are opened when the ebb-tide is on, and shut before the flux sets ill, so that the level of the lake of Haarlem is kept at a con- siderably lower elevation than that of the adjacent sea. Every landowner contributes his proportion towards the maintenance of these indispensable ducts and defences. They are like so many sailors in a leaky vessel, com- pelled to work at the pumps day and night to prevent her from going down,-From Notes made on "late visit.