There is another mode, adopted in Petersburg, of building under water by driving the piles and cutting them over level with a machine, and then sinking caissons of brick-work. This mode, however, is un suitable for sea walls in general, which ought to be founded as deep as possible, and, at all events, un der the bed of the water. The motion of the Neva is so slow as hardly to render this worth attending to.
In Britain, such walls are fortunately rare; for in proportion as it is agreeable and flattering to self love to protect or gain lands never before cultivated, it must be mortifying to be obliged to protect sdch as have long been subjected to agriculture, and where success can only be said to have a negative advan tage.
Embankments for firing Drifting-sands, Shells, or Mud.—In several tracks of coast, the sea at ordinary tides barely covers a surface of sand ; and these sands in dry weather, during high winds, are drifted and blown about in all directions. Great part of the north shores of the Solway Frith, of Lancaster Bay, and of the coast of Norfolk, is of this description. Mr Young, in his Farmer's Letters, informs us, that a considerable part of the county of Norfolk was drift-sand, even as far inland as Brandon in Suffolk, before the introduction of the turnip culture ; and Harte (Essay I.) states, that some of what is now the richest land in Holland, was, about the middle of the sixteenth century, of this description. The sug gestion of any mode, therefore, by which, at a mo derate expence, such tracks could be fixed, and co vered with vegetation, must be deemed worthy of a pace in this article.
The mode which nature herself employs is as fol lows : After the tides and wind have raised a marginal strip of sand as high as high water mark, it becomes by degrees covered with vegetation, and chiefly by the Elymus arenarius, Pritieitm junemmt, various spe cies of Janette, and sometimes by the Galium errata. With the exception of the first of these plants (the leaves and stalks of which are manufactured into mats and ropes in Anglesea, and the grain of which is ground and used as meal in Ireland), they are of no other use than fixing the sands, which, being coin. posed in great part of the debris of shells expand as they decay, and contribute to, raising the surface still higher, when the fibrous roots of good grasses soon destroy the others.
To assist nature in fixing drift-sands, it is only ne cessary to transplant the Elymus, which is to be had in abundance in almost every sandy coast in Bri tain; and as it would be liable to be blown away with the sands, if merely inserted in the common way, it seems advisable to tie the plants to the up per ends of willow or elder rods, of two or three feet in length, and to insert these in the sand, by which means there is the double chance of the grass growing, and the truncheon taking root. The elder will grow exposed _to the sea breeze, and no plant throws out so many and such vigorous roots in pro portion to its shoots.
The mode by which such sands were fixed in Hol land was by the formation of wicker work em. bankments, and by sticking in the sands branches of trees, bushes, furze, &c. in all directions. These obstructed the motion of the sands, and collected masses of sand, shells or mud, and sea-weeds around them, which were immediately planted with some de scription of creeping grass ; or, what was more fre quent, covered with a thin coating of clay, or allu vial earth, and sown with clover.
Though the most certain and least expensive mode of gaining such lands be undoubtedly that of second ing the efforts of nature, by inserting bushes, and planting the Etyms in this way, yet it may some times be desirable to make a grand effort to protect an extensive surface, by forming a bank of branches, which might, in a single or in several tides, be filled with sand and shells. It is evident, that such a bank might be constructed in various ways; but that which would be most certain of remaining firm, and effecting the purpose, would be one regularly constructed of framed timber, the section of which would resemble a trussed roof; each truss beingjoined in the direction • of the bank by rafters, and the whole inside and surface stuck full of branches. To retain it firm, piles would require to be driven into the sand, to the upper parts of which would be attached the trusses. The height of such a barrier would require to be several feet above that of the highest spring-tides ; and the more its width at base exceeded the proportion of that of an equilateral triangle the better.