642
THEORY AND PRACTICE OP ENGINEERING.
Book II.
• or formed into islands elsewhere; the quantity of earthy matter depends upon the natureof the river that discharges itself.
The Rhone , which passes through the Lake of Geneva , there deposits much of the matterit brings down from the glaciers of Mont Blanc, but it acquires in its after-course a con-siderable accession from the tributary streams of the Alps of Dauphiny. This river poursitself into the Mediterranean, which it discolours for a distance of 7 miles from its mouth,depositing a fine sediment in horizontal strata along the whole shore over which itswater spreads: since the time this coast was under the dominion of the Romans itsharbours have been silted up, and are now a league from the shore.
The Po has poured out so much sediment, received from the Alps and Pyrenees , thatwhere it is discharged in the sea not under the influence of a tide, it has added to the coast,for a length of 100 miles, a breadth of land in some places as much as 20 miles. Adria, atown of importance, and which gave the name to the sea that washed its walls, is now 20miles within land; this increase of land has been more rapid as the embankments of thevarious rivers have l>cen elevated.
The Nile, which discharges 250 times as much water as the Thames , deposits its detritusin its course over the fertile lands of Egypt , which it irrigates; therefore its delta is smallin comparison with that formed by other large rivers. The land of Egypt is calculated tohave been raised above 6 feet in height since the commencement of the Christian a.*ra; andthe earth deposited is about half argillaceous, a fourth carbonate of lime, and the remaindercarbonate of magnesia and oxide of iron. The delta of the Nile at its termination has adepth of sea of 2000 feet or more. The seven mouths of this noble river are no longeropen ; five of them are silted up, as is the Lake Marreotis.
The Mississippi empties itself into the Gulf of Mexico after a course of 3000 miles, andoften brings down with it whole forests, and large quantities of alluvial matter.
The Ganges and Rrahmapoufra, which bring tlie water from the Himalaya mountains, aredischarged into the Bay of Bengal; the delta at the mouth reaches 200 miles along thecoast, and commences 220 miles from the sea. The tide extends its influence to the headof this delta when the river is low, and the sea for a distance of 60 miles in times of Hoodsis discoloured by the alluvial matter, which chiefly consists of sand and silt borne bythe tidal current to a distance of 400 miles. In one direction a tract of land 40 milessquare and 114 miles in depth has been washed away in the course of a few years;and it has been stated, on the authority of Major ltennel, that the deposits forming theSunderbunds equal in extent the area of the whole of Wales .
The Delta of the Niger stretches 300 miles along the coast, and extends 170 miles into theinterior.
Quantity of Water discharged by Rivers . —This is found to be the annual produce of the rainthat falls over the several districts in which they act as a natural drain, after deductingabout \ for filtration and evaporation, and the total expenditure of a river may be deducedby the products of its mean section and mean velocity. Button estimated the area of allthe land to be equal to 63,728,938 square miles, and the mean quantity of rain to be equalto 36 inches ; if then we calculate the quantity carried down by such a river as the Po, weshall find that it would be necessary to have 1400 such rivers; the Po having a meanbreadth of 1000 feet, and its waters moving with a velocity of 4 miles an hour.
'Hie annual discharge of the Rhine at Bayle is estimated at 1,046,763,676,000 cubic feet.The Tay at Perth, in Scotland, 100,000,000.000 cubic feet. The Thames , which drainsupwards of 5000 square miles, according to Dr. Halley’s observation made at KingstonBridge, 239,765,120,000 cubic feet per annum ; but it is doubted whether any of thesecalcinations are to be depended upon.
Sand Banks. — The Dogger Bank is 350 miles in length, and that in the tirth of Torth110 miles, and their average height is about 80 feet. The area of these two banks is about£ of the German Ocean, and almost equal in extent to 5 of England and Scotland ; theyare formed of sand mixed with broken shells and corals, and have been thrown up by thetidal current.
Downs or Dunes are formed on a low coast, where the bottom is composed of sand; thisbeing driven by the waves towards the shore is left dry on every reflux of the tide; thewinds which blow from the sea then carry them inland, and form sandy flats and hills.These sometimes occur at the mouths of rivers, and such a sand flood, as it is termed,effectually stops the passage of the water, driving it into some new channel.
The Beach may be divided into three portions: viz. that comprised between high w’aterof spring and neap tides; that between high water at neaps and the extreme low* water ofspring tides ; and the expanse of sand and gravel, of greater or less extent in proportion tothe flatness or steepness of the shore, never laid dry, but covered at the lowest ebb byshallow water. These are termed high water, low water, and littoral sea beaches. Thefirst is usually composed of shingle or round pebbles, thrown up more or less steep. Thesecond or low water beach usually presents a gently sloping surface, strewed with pebblesor patches of tand.