Should the wooden piers be filled in with rough stonework, the • latter material will be retained by means of close boarding laid hori zontally against the upright posts of the framing; and when that planking is carefully laid, the interior may be filled in with shingle, sand, or clay, or with concrete made with a hydraulic lime basis. It would appear that the best position for the planking, under these circumstances, is upon the outside of the frames ; for although unquestionably, the planks would resist the tendency to displacement, from the weight of the filling-in material, more efficaciously if they were nailed upon the inside than if they were nailed upon the outside, yet in the latter case the framing would be far more efficaciously protected from the action of the waves, and it would be much easier to repair any accidental damage. The essential condition in all theso cases le, in fact, that the substantial framing should be withdrawn as much as possible from the direct action of the sea ; and that those parts, which are exposed to it, should be' susceptible of being easily replaced.
The advantages of wooden jetties consist in the economy of their construction ; but in practice it is found that the repairs of such work are so frequent, and so costly, that the use of wood should be only resorted to when it is utterly impossible to resort to the use of more durable, even though originally more expensive materials. Open wood piers have, however, another occasional advantage, namely, that they do not seriously interfere with the advance of any littoral current, and therefore they are not so likely to produce any change in the outline of the shore, by producing a deposit of alluvial matters, as a solid pier would do. In the landing stage lately erected at Margate an attempt has been made to unite the advantages of open piers with greater durability, by the use of cast iron piles ; but It would be desirable to watch the effect of sea water on the iron during some years, before passing any opinion on the merits of this system. It may be added that the use of creaaoted timber, for works exposed to the action of the sea, seems to have removed some of the most serious objections to the use of that material ; for not only has this process Increased the durability of the wood. but it would appear to constitute also an efficient protection against the teredo. Whether wood or iron piles be used, it is necessary to guard against the percussive action of the ground swell upon their heads; and it is for this purpose that the layer of concrete, rubble, or fascines, over the foundations is introduced.
Stone piers are either executed of loose rubble thrown down and allowed to assume its natural slope by the action of the waves ; or of ashlar masonry external walls bonded together so as to form a series of cases filled in with concrete, or with rubble masonry, or with a hearting of earthwork protected by dressed masonry walls; the determining motives for the selection of either of these modes of execution being, of course, merely economical ones. The most difficult part of such
structures consists in their foundations when executed at some distance from the shore, and in deep water, where caissons, or coffer dams cannot be used ; especially if any strong littoral currents should exist in the particular locality. The very successful mauner in which the new graving dock of Toulon was entirely constructed of concrete, under a head of water of not leas than 37 feet, proves that it would be p asible to make ti much greater use of that material than is usually Gone in England ; but of whatsoever material the piers may be formed, the necessity fur protecting the foundations from the undermining actions of the currents, or of the ground swell, must remain the name.
The other practical remarks to be made with respect to the execution of masonry piers are, that they must settle equably throughout their whole tunas; that no openings should be left by means of which water could get into the interior, either directly, or by syphonment; that the masonry should be protected against the abrasion of the allu vium carried forward by the currents, and that the ships lying alongside the piers should not bear directly against the face of the masonry, for which purpose fender piles are used ; finally, all abrupt projections must be avoided whether in the plan or the elevation of the piers, and the surfaze of the roadway must bo carefully laid, so as to throw off all the water which may break over the piers. It is usual to make the width of the roadways. in the body of the work, from 10 to 23 feet, and to increase that width, at the heads, to from 30 to 50 feet ; the body of the pier being kept about 10 feet above the level of high spring tides, and the heads about 12 feet above the same level. Generally speaking, a lighthouse is erected near the extremity of a pier; fog bells and signal guns are fixed ; and the necessary bollards, or mooring posts, reeving pullies, capstans, and landing staircases are to be provided. It is curious that the softer calcareous atones are nearly as much affected by the boring molluscs as wood is affected by the 'imperil% and teredo ; and that the solenidaa, pholidx, and litho demi, exercise fearful ravager' upon that class of stones in some places. Where those animals abound, or where the littoral currents carry forward much alluvial matters, it become. necessary to employ the more resisting siliceous rocks, or the granites.