648 PRACTICE OF AGRICULTURE. Part III. 



of floods, will necessarily ooze through a wall of this construction. The water so col- 

 lected is let through the wall by tubes, or tunnels of boards 515 



{Jig. 5l5.)i with a valve opening outwards on their exterior^ 



extremity. When the flow of water from without approaches, I \ 



it shuts the valve, which, remains in this state till the flood sub-^"^ 



sides, when the height of the water within being greater than that without, it presses open 



the valve and escapes. Walls and valves of this kind are common enough in the drier 



parts of the fenny districts of Lincolnshire and Cambridgeshire. 



4018. The earthen mound (Jig. 516 

 516.) is the most general descrip-_ 

 tion of embankment, and as it isl 

 executed at considerable expense, is ^ 

 only undertaken by such as have a "^^^ 

 permanent interest in the soil. This barrier applies to sea lands overflowed by every spring 

 tide, and to alluvial plains inundated by every flood. It is set out in a direction parallel 

 to the shore, and to the general turns of the river, but not to its minute windings ; and 

 it is placed farther from or nearer to the latter, according to the quantity of water in time 

 of floods, the rapidity of the current from the declivity of the bed, the straight course of 

 the stream, and the intended height of the bank. The two sides of such a mound are 

 generally formed in different slopes. That towards the land is always the most abrupt, 

 but can never be secure if more so than 45 ; that towards the water varies from 45 to 

 15 ; the power of the bank to resist the weight of the water, as well as to break its force 

 when in motion, being inversely as its steepness. The power of water to lessen the 

 gravity of bodies, or in other words, to loosen the surfaces over which they flow or stand, 

 is also lessened in a ratio somewhat similar. 



4019. The formation of the earthen mound consists merely in taking earth from the 

 general surface of the ground to be protected, or from a collateral excavation, distant at 

 least the width of the mound from its base line, and heaping it up in the desired form. 

 The surface is then in general cases covered with turf, well rolled in order to bind it to 

 the loose earth. The earth of such mounds is generally wheeled by barrows ; but some- 

 times it is led by carts placed on a wooden roller instead of wheels, which, with the 

 treading of the horses, serves in some degree to consolidate the bank. 



4020. The excavation serves the same purposes as the open drain in the earthen wall ; 

 and similarly constructed sluices or valves are introduced on a larger scale. Some- 

 times also the interior water is drawn off by windmills, and thrown over the mound into 

 the river. This is very common in Huntingdonshire, and might be greatly improved on 

 by employing steam-engines for entire districts, one of which, of a ten horse power, would 

 do the work of twenty mills, and this in calm weather, when the latter cannot move. 



4021. Embankments of this description are the most universal of ani/ , and their sections 

 vary from a scalene triangle of ten feet in base, and three feet in height, as on the Forth 

 near Stirling, and the Thames at Fulham, to a base of 100 feet, and a height often feet, 

 as in the great bank of the Ouse, near Wisbeach. The great rivers of Germany and 

 Holland are embanked in this way, when so far from the sea as to be out of the reach of 

 the tide ; as the Vistula at Marienwerder, the banks of which, near Dantzic, are above 

 fifteen feet in height ; the Oder, the Elbe, &c. All these banks are closely covered in 

 every part with a grassy surface, and sometimes ornamented with rows of trees. 



4022. But near the sea, where such banks are washed by every tide when the course of 

 the wind is towards the shore, and by all land-floods and spring-tides, grass is only to be 

 found on and near their summits. The rest of the bank is bare, and to preserve it from 

 the action of waves, currents, and the stones, pieces of wood, and other foreign matters 

 which they carry with them, the surface is covered with gravel, reeds, or straw, kept down 

 by pieces of wood ; faggots, wicker hurdles, nets of straw ropes, straw ropes laid side by 

 side and fastened, or handsfull of straw fixed in the ground with a dibber {Neale's Travels 

 inGermany, ^c. chap, i.), or any other contrivance, according to the situation, to prevent 

 the washing away of the bank. It is common to attribute to these coverings the poweir 

 of breaking the force of the waves ; but this power depends, as we have already stated^ 

 on the slope of the bank and its smoothness ; and the use of the surface covering, and 

 of the constant attention required to remove all obstacles which may be left on it by floodis 

 and tides, is to prevent the loosening power of the water from wearing it into holes. 

 JFor this purpose, a sheet of canvass or straw-netting is as good, whilst it lasts, as a 

 covering of plate iron or stone pavement. 



4023. J[ll banks whatever require to be constantly watched in time of foods or spring- 

 tides, in order to remove every object, excepting sand or mud, which may be left by the 

 water. Such objects, put in motion by the water, in a short time wear out large holes. 

 These holes, presenting abrupt points to the stream, act as obstructions, soon become 

 much larger, and if not immediately filled up, turfed over, and the turfs pinned down^ 

 or the new turfs rendered by some other means not easily softened and raised up by the 



