seas; or, the flowing down and accumulation of the surface waters from the surrounding and more elevated districts. Lakes formed by the first of these causes being constantly fed and replenished, may be regarded as permanent reservoirs; and those formed by the second are dependant upon the preservation of their inlet from the ocean; but those which receive their supply from the drainage of the lands around, appear destined to extinction by the constant deposit within them of the solid matters brought down by the water. Thus, the Black Sea, the Caspian, and Arral, are fairly supposed to have originally formed one vast lake, the ridges in which have now become elevated, so as to form permanent boundaries between them. The Caspian, also, has evidently become reduced in extent, as proved by the marine matters now found at a distance from its present shores.

80. Fresh-water lakes, of considerable extent and little depth, are sometimes worthy of being entirely drained for the sake of cultivating the site they occupy. One of the most recent examples of this class of works is the drainage of the Lake of Haarlem in Holland. This lake is situated between Leyden and Amsterdam, and communicates with the Zuyderzee. The bottom of it consists of a rich alluvial deposit well fitted for agriculture. A Dutch engineer, popularly known by the name of "Leeghwater," or "drier up of water," formed a project for draining this lake in 1623, and another proposal for the same object was brought forward at the end of the last century, when steam was first employed in draining; similar works having been already executed in the Beilm and Diem. The area of the Lake of Haarlem is equal to 45,230 acres, and its average depth about 14 ft., the cubic contents being

equal to 800 millions of tons of water. The longest side of the lake is parallel to the sea, and separated from it only by a very narrow strip of land. When drained, the bed of the lake will constantly require mechanical power to keep it dry. Observations, continued during a period of 91 years, show that the maximum quantity of rain which falls upon the lake amounts to 36 millions of tons of water monthly. The Dutch government having appointed a commission of engineers to report upon the best means of draining the lake, many proposals were submitted and examined, and it was ultimately determined to adopt the plan recommended by Messrs. Gibbs and Dean. These gentlemen employ three engines for the purpose of draining the lake, each being of great power, whereby the total current cost is much less than would be incurred by using a greater number of smaller engines. These three engines are named the "Leeghwater," the "Cruquius," and the "Van Lynden," after three celebrated men of these names, who had interested themselves in the draining of the lake.

81. Of these three engines the "Leeghwater" was first erected, with suitable houses and pumping machinery. The first step in this work was to construct an earthen dam of a semicircular form, inclosing about 1 acre of the area of the lake, and adjoining its bank. The space inclosed by this dam was then cleared of water by a small steam engine, and the foundations for the houses and machinery commenced. These foundations consisted first of 1400 piles, which were driven to the depth of 40 ft., into a stratum of hard sand. Upon these piles, and at the depth of 21 ft. below the surface of the lake, a strong platform was laid, and upon this a wall, pierced with arches, was constructed, at the distance

of 22 ft. from the intended position of the engine-house. Upon this wall a thick flooring of oak was laid, between the wall and the engine-house. The pumps rest upon the platform, beneath and opposite to the arches, and their heads pass through the floor just described, standing about 3 ft. above its level. Into the space left between the engine-house and outer wall, the water raised by the pumps is received and discharged from it on either side of the boiler-house, through sluice gates, into the canals conducting to the sea sluices. The general arrangement of the engine, boilers, pumps, and sluices will be understood from Fig. 30, in which a represents

the engine; B, the boiler-house; cc, the pumps; and

D D, the sluices through which the water is discharged. The engine has two steam cylinders, one within the other, united at the bottom, but with a clear space of 13 in. between them at the top under the cover, which is common to both. The large cylinder is 12 ft., and the small one 7 ft. in diameter. The small cylinder is fitted with a piston, and the large cylinder with an annular piston. These pistons are connected by one main piston rod (of the internal cylinder) 12 in. diameter, and four small rods (of the annular piston) 44 in. diameter each, with a great cap or cross-head, having a circular body 9 ft. 6 in. diameter, and formed to receive the ends of the balance beams of the pumps. The pumps are eleven in number, and each of them 63 in. diameter, with a cast-iron balance beam turning upon a centre in the wall of the engine-house, one end of which is connected with the great cap of the engine, the other to the pump rod. Each pump rod is of wrought iron, 3 in. diameter, and 16 ft. long, with an additional length of 14 ft. of patent chain cable attached to the pump piston. The steam and pump pistons have a stroke of 10 ft. in length; each pump is calculated to deliver 6.02 tons of water per stroke, or 68.22 tons for the eleven pumps. The quantity actually raised is found to be about 63 tons. The action of the engine is as follows:-The steam being admitted, the piston and great cap are thereby raised, and the pump pistons make their down stroke. At the top of the steam stroke a pause of one or two seconds is made, to enable the valves of the pump pistons to fall out, so that, on the down stroke of the steam piston, they may take their load of water without shock. In order to sustain the great cap and its dead weight during this interval, an hydraulic apparatus is brought into use, which

consists of vertical cylinders, into which water is admitted, forcing upward two plunger poles which sustain the cap, the water being prevented from returning by spherical valves fitted at the lower part of the cylinders. The arrangement of the two steam cylinders is adopted in order to bring the load under immediate command, the varying character of which would otherwise require occasional alteration of the dead weight to overcome it, which would involve great delays and inconvenience. By the use of the two cylinders, the dead weight raised by the small piston does not usually exceed 85 tons, the extra power required being derived from the pressure of the return steam at the down stroke upon the annular piston. A skilful regulation of the expansion and pressure of steam in the small cylinder thus enables the engine-man to provide for all cases of difference of resistance without the delay of altering the dead weight. Respecting the power of the "Leeghwater," it appeared from experiments conducted by a sub-committee of the Commission, that the engine will do a duty equal to raising 75 million pounds, one foot high, by the consumption of 94 lbs. of good Welsh coal, and exerting a net effective force of 350-horse power. The lift being 13 ft., the engine works the eleven pumps simultaneously; the net weight of water lifted being 81-7 tons, and the discharge 63 tons per stroke. When the site of the lake is cultivated, the surface of the water in the drains will be kept at 18 in. below the general level of the bed; but during floods the waters of the upper level of the country will be raised above their usual height, and the lift and head will be increased to 17 ft. To test the power of the engine to meet these cases, the eleven pumps were worked simultaneously, without regard to economy of