542

HISTORY OF ENGINEERING.

Book T.

that on an average upon every 1000 acres, 7,260.000 cubic feet of water are annually raisedand carried off; and a ten-horse steam engine performs this work in less than 232 hours.

The quantity of rain is computed at 26 inches ; and if 3 inches fall in a month, allowing

1 inch or ^ to be evaporated or absorbed, we shall have 7260 cube feet of water to the acreto be carried away by means of machinery, before the land can be rendered fertile.

The high lands which pour their waters over these fens comprise not less than 12,000acres, and during a rainy season, as much as 40,000 cubic feet of water is sent down fromthem per minute, which is considerably augmented by that from the fens. This vastquantity was impeded in its passage to the sea by the narrowness of the three sluices throughwhich it had to pass, viz. the Austins, which had an opening of 14 feet, Maudfoster, 13 feet,and Tichtoft, 4 feet. When the late Mr. Rennie was called in to survey and improve thisextensive district, he commenced his operations upon a bold and noble scale : he shortenedthe course of all the rivers, made new cuts and drains in various directions, gave their bedsan inclination of from 3 to 5 inches in a mile, formed a catch-water drain at the skirt of allthe high grounds around the fens, and conducted the upland waters by an inclined bed of6 inches to a mile, through a separate outlet into the head of the Eau Bank cut, into whichall the drainage waters were conducted.

Thus the Great Level, as it is commonly called, is now more effectually drained, and oncemore redeemed from the state into which it had fallen after the dissolution of the religioushouses in the time of Ilenry VIII., whose fraternities had paid great attention to keepingopen the passages for the water, and the repair of the several banks, although not sufficientlyaware of the advantages arising from opening and cleansing the natural outfalls of the drains,which the daily flowing of the tides choked up. It would be exceedingly advantageous ifsuch a level could be placed under one direction, and not governed by persons interested inone district alone, and that, perhaps, the most remote from the point at which the watersare discharged into the ocean : no management could be effectual or beneficial to the wholebody of proprietors, but one which should be established upon a system embracing theentire district to be drained.

Accurate maps and levels, meteorological observations, and the geological structure of thedifferent portions of the district, together with a knowledge of the machinery in use forevery kind, of hydraulic work, should be acquired by the board, or the individual chargedwith so important and often costly work.

In the middle ages, before science was called in to assist at these operations, the ruralengineer was content to confine the rivers with embankments of earth, raised 5 feet abovethe level of the land adjoining -, these had sometimes a base of 18 feet, and a surface at topthe same as the height : others with the same base and 6 feet high, had a width of 12 feetat the top, and these were calculated to keep out water in almost any situation: towardsthe water they were usually clayed to 1^ feet in thickness, not only up the side, but over theentire top.

Sea banks were made 12 feet in breadth at top, and carried up 2 feet in height above thehighest tides: these were sloped and turfed, and strengthened in various places with stakes,piles, and timber: sometimes rows of piles were driven parallel to the river at distances of

2 and 3 feet apart; and after uniting their heads by a plank, or weaving rods around them,the parallel spaces between were filled up with chalk or some other hard substance : thusthe side of a Avail towards a tidal river represented so many steps of piling and chalk, whichfrequently became more tightly bound together by the sea-weed. At times this methodof defending the shore would be swept away altogether by the force of the tides whenacted on by the winds, and a frightful breach would be the result, inundating extensivedistricts, and destroying property to a great amount.

Banks were sometimes made of sand, with twigs of the broom placed in horizontal layers,which, when properly clayed on the surface, and turfed, were found to stand in manysituations remarkably well.

Loughs Swilly and Foyle, in Ireland . The first of these lakes has a communication withthe Irish Channel by a narrow course, above which it spreads over a considerable tract ofland, and afterwards narrowing again its breadth, joins the river Foyle about 4£ miles abovethe town of Londonderry , where the channel is deep enough to allow the navigation of vesselsof 600 tons burthen.

Lands of 25,000 acres, overspread by the tide, all alluvial, and gradually deposited, wereundertaken to be reclaimed under the direction of Mr. Macneil, by the construction of asea wall 14 miles in length.

Lough Swilly, from its width at the mouth, is more subject to be acted upon by thewinds than Lough Foyle, and the highest tides rise 18 feet: here the first embankments weremade, which reclaimed about 2000 acres. They were commenced as follows : —the soundingsbeing accurately taken, the wall was set out in the most favourable position ; a tide gaugewas established, on which was permanently marked the height of high and low water. Theslopes given to the embankment varied on the side towards the sea from 3 and 4 to 1, and