E MB

374

E MB

materials, which, as has been seen, is essential in bold steepbanks or shores, it is necessary that they be laid in propermaterials, and be closely jointed next the sea, or the rivers,so as to be fully capable of resisting the entrance of water.Great care is requisite in doing this, or the bank will not stand,for the water, insinuating itself between the openings, will sinkdown among the stones, softening and loosening the clayey orearthy matters underneath, by which, portions of them will| be forced out and washed away. Hollows being formed in! that way below, the stones naturally sink down ; and the| waters rushing into the cavities with considerable impetuosity,

| quickly displaces others, and the whole embankment is soon! destroyed. This very frequently takes place with the headsthrown across rivers, and such paved or causewayed banks asare formed with the view of protecting and preserving bold! and open shores. Such shores are especially liable to bei undermined and carried away by the washing operation ofthe waters which come against them. In order to render theembankments perfectly secure in such cases, they should be! laid with good mortar, and be pointed with a strong cement.A good coat of gravel, in some cases of this kind, is evenfound far superior to paving with stones.

It sometimes happens that rivers near their mouths formshallow estuaries, and occupy much ground which might beusefully employed. In this case an entirely new outlet maysometimes be made, through which the river may at oncedischarge itself into the sea; and the whole course willprobably be soon filled up by the deposition of soil and mudbrought in by the tides ; for it is the current which clearsthe channel, and when this is taken away the channel soonfills up. In the course of a short time the old mouth of theriver will be so filled up as scarcely to admit the tide; andan embankment across it may lay a large fertile tract of landquite dry.

In constructing embankments of the quay, or other similarkinds, a mortar formed from powdered unburnt lime-stoneand coarse sharp sand is employed; the whole being pointedwith puzzolana earth, by which they become as solid as rock,and fully resist the effects of water. The lime of particularsorts of lime-stone is found more proper for forming this sortof mortar-cement than that of others : thus, that found atDorking, in Surrey , is supposed to constitute the mostdurable substance of this kind of any in the kingdom ; andhas been employed in many works near London . And anexcellent sort of lime-stone, for the same purpose, has like-wise been discovered near Worsley, in Lancashire , which isthere termed Sutton lime.

An excellent cement for this use, which hardens underwater, may be composed by having four parts of blue clay,

| six of the black oxide of manganese, and nine of carbonateof lime, submitted to a white heat, and then well incorporatedwith sixty parts of sand, and as much water as may be neces-sary, to form 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 easy and gentlydeclining slopes from their beds to their borders or banks,and those which are formed in a steep upright manner, ofrocky materials, such as are shown at Figures 3 and 4, arethe least exposed to injury from the effects of the waters: thetwo former being the most secure when spread over orcoated with good coverings of sand or gravel, or uniformlyturfed over quite down to the water-side with the sward of atough old pasture. The strength and firmness of their banksare in proportion to the extent of the slope ; and their dura-bility depends on that of their being made uniform on, theirsurfaces, both in respect to hardness and smoothness : as, inthe former case, from the great length of slope, the flows and

decreases of the waters act more momentarily on 4 ] ie w, n fcs

ferent parts, and their greater weight renders tu ^ ^eirmore firm ; while, in the latter case, by the equality ^ 0 nesurfaces, the power of the water is rendered the sam ^ ofpart as another, and no obstacles are left for the pr° t j ie01 .eddies, or other means of forming holes or breaks m ofIn the latter, or those of the bold, upright, rocky ^ t j, ebanks, their strength chiefly depends on the resistan ^ a otlarge quantity of materials by which they are backed; ^ ovBi eton the manner in which they are disposed, as in 41 „ tn es9case ; and their durability, on that of the uniform c0 ® j^g : as,of texture in the parts opposed to the effects of the wa ^where these have fissures in them, or are softer m hreakdV 11than others, the waters are liable to enter and „mre of tbethe banks in time, according to the particular natucases. forlD3 of

It is, therefore, of importance that the modes ana ^embankment, which are thus naturally presented, s i ffer e

- ,h« if »!»«

nd

improved upon by art. It is evident,formed behind the embankment, as in Figureletter x, the shores or banks, though, in this case, as - _ aD idetached from the land, would be found equally str0 .^ : \beticapable of resisting the pressure of the waters, aS , aI) doriginal state. Hence, if a mound or bank were for ^placed out at the distance of one, two, or three miles ot jjershore or other embankment, within the bed of thewaters, as at y in the same Figure , it would b jcapable of resisting them as in the former instance, jtsmore liable to be broken down by their pressure t j e telyformer station ; and would also defend them as c0 ^. 0{0 tb efrom the intermediate space of land, as it did before tra ctsnarrow trench. Consequently, on this principle; v 0 ^taia e ®of land may, in different parts of the kingdom, heby judicious embankments. , a fFor a

Though the shores of bold steep coasts may n ^ van ta? eexamples equally capable of being followed with a ^ tl> 6as the above, they nevertheless suggest useful hm g [ )0 reS,purpose of defence, in cases of bold, abrupt, brok ^ s tanc eSconstituted of earth, or of that material and rocky s that^fintermixed. It readily presents itself to the min ,raising a good perpendicular stone wall against s by

renders them nearly as strong and lasting as those ^ ^ a pknature of steep solid rocky bodies. This sort of wa b eis exhibited at Figure 6 ; but though this rnet ”° ( j va jitag e ’practised, in cases of the above kind, with great . gs,

it is not by any means applicable in general to rI gta0 dwith them, the waters, during the periods of . ; n g tb e ’ rneed of room to spread, which is the great use of g' ^ e th ebanks a sloping form ; while, in this way, it won fb eeffect of doing more injury than was the case j> e g0 coH"increased rapidity of the current caused by its be »fined, doing greater damage to the banks. I® 8 a j t li V v °' thowever, happen in which it may be had recourse wipriety, in defending a part of the bank of a n ’ t ofgiving it a sloping direction, or for protecting ° n ® w e ,bank at the risk of that which is opposite to it a tteb^ econstructed piers, in such cases, are preferable, an ^ n ,gywith less expense to maintain. But instead of the^ ’ r fect'Hsuggest one that may answer in some respects m °of earthy j,as, in place of bringing together such a mass . n<r sU cother substances, as may be proper for consti g( j v a®banks as are shown at Figures 1 and 7, it may be rn pigiifitageous to have one formed, such as is shown base;, 9 ^the side of which, next the water, forms, with 1 be al9 aaangle of about 45 degrees. This will be capab e _ g ;jjly .all the weight or pressure of water that can P^ ^ e %c e Pbrought upon it, equally well with that of Figur