It sometimes happens that rivers, near their mouths, firm shallow estuaries, and occupy much ground which might be usefully employed. In this ease, an entirely new outlet may sometimes be made, through which the river may at once discharge itself into the sea ; and the whole course will, probably, be soon tilled up by the deposition of soil and mud brought in by the tides ; for it is the current which clears the channel, and when this is taken away the channel soon tills up. In the course of a short time the old mouth of the river will he so filled up as scarcely to admit the tide ; and an embankment across it may lay a large fertile track of land quite dry.
In constructing embankments of the quay, or other similar kinds, a mortar flamed f ens powdered unburnt lime-stone and coarse sharp sand is employed ; the whole being pointed with puzzolana earth, by which they become as solid as rock, and fully resist the effects of water. The lime of particular sorts of lime-stone is found more proper for forming this sort of mortar-cement than that of others : thus, that found at Dorking, in Surrey, is supposed to constitute the most dura ble substance of this kind of any in the kingdom ; and has been employed in many works near London. And an excellent sort of lime-stone, for the same purpose, has like wise been discovered near Worsley, in Lancashire, which is there termed Sutton lime.
An excellent cement for this use, which hardens under water, may be composed by having four parts of blue clay, six of the black oxide of manganese, and nine of carbonate of lime, submitted to a white heat, and then well incorpo rated with sixty parts of sand, and as much water as may be necessary to firm it into a mortar. .See CONCRETE.
It is invariably found, in examining the shores of the sea, and the banks of rivers, that such as have easily and gently declining slopes from their beds to their borders or hanks, and those m hich are formed in a steep upright inanner, of rocky materials, such as are shown at •'iyures 3 and 4. are the least exposed to injury from the cilia:Ls of the waters : the two former being the most secure when spread over or coated with good coverings of sand or gravel, or uniformly turfed over quite down to the water-side with the sward of a tough old pasture. The strength and firmness of their hanks are in proportion to the extent of the slope ; and their durability depends op that of their being made uniform on their surfaces, both in ctspect to hardness and smoothness : as in the former case, 'own the great length of slope, the flows and decreases of the waters act more momentarilv on their diffTent parts. and their greater weight renders their banks more limn ; while, in the latter case. Icv the ((quality of their sot thees, the power of tint water is t endured the same on one part as another, :it'd 110 obstacles are left for the producing of eddies, or other means of forming lades or breaks in them.
lit the latter, or those of the bold, upright, rocky kind of batiks, their strength chiefly depends on the resistance of the large quantity of materials by which they are hacked, and not on the manner in which they are disposed. as in the former ease ; and their durability, on that of the unif4m compactness of texture in the parts opposed to the effects of the waters: as, where these have fissures in them, or are softer in some parts than others, the waters are liable to enter and break down time banks in dine, according to the particular nature of the cases.
It is, therefore, of importance, that the modes and forms of embankment, which arc thus naturally- presented, should be improved upon by art. It is evident, that if a cut were formed behind the embankment, as in Figure 5, at the letter x, the shores or banks, though, in this case, as it were, de tached front the land, would be found equally strong. and callable of resisting the pressure of the waters, as in their original state. hence, if a mound or bank were formed. and placed out at the distance of one. No. or three miles from the shore or other embankment, within the bed of the sea or other waters, as aty in the same Figure, it would be equally capable of resisting thou as in the former instance, and not more liable to be broken down by their pressure than in its former station ; and would also defend them as completely from the intermediate space of land, as it did from the narrow tiench. Consequently, on this principle, vast tracts of laud may, in different parts of the kingdom, be obtained by judicious embankments.
Though the shores of bold steep coasts may not afford examples equally capable of being followed with advantage as the above, they nevertheless suggest useful hints for the purpose If defence, in eases of bold, abrupt, broken shores, constituted of earth. or of that material and rocky substances intermixed. It readily presents itself to the mind, that the raising a good perpendicular stone wall against such banks, tenders them nearly as strong and lasting as those formed by nature of steep solid rocky bodies. This sort of walled bank is exhibited at Figure 6 ; but though this method may be practised, hi cases of the above kind, with great advantage, it is not, by any means, applicable in genet al to rivers; as, with them, the waters, during the periods of floods, stand in need of room to spread, which is the great use of giving their banks a sloping form ; while, in this way, it would have the Ilk-a of doing more injury than was the ease befOre. The increased rapidity of the current, caused by its being so con futed, doing greater damage to the banks. Instances may, however, happen in which it may be had recourse to m ith propriety, in defending a part of the bank If' a river, m ithout giving it a sloping direction, or for protecting one part of a bank at the risk of that which is opposite to it ; but well constructed piers, in such eases. are preferable. and attended with less expense to maintain. But instead a these, t may suggest one that may answer iu some respect nuire perfectly; as, in place of bringing together such a mass of earthy or other substances, as may be proper for constructing such banks as are shown at Figures I and 7, it may be more ad vantageous to have one firmed, such as is shomn at Fvut e 8, time side of which, next the water, forms, with the base, an angle of about 45 degrees. This will be capable of bearing all the weight or pressure of water that can possibly be brought upon it, equally well with that of Figure 1, except that the operation of the tides would break the superficial part of the side next the sea, unless prevented by coating it with some durable substance, such as paving stones, bricks, or other similar materials.