The width to be given to the channel between the two piers of harbours, where such double works are required, should be at least such as to allow three ships to enter at the same time, under full sail ; and this width may vary between 100 and 250 feet, according to the class of vessels frequenting the harbour. At the heads of the piers the widths of the channels should be increased, because the ships entering require more room for their evolutions when under the influence of the way they bring in from the open sea than they do when they have followed the narrow channel for some time. The general use of steam-tugs, by enabling several vessels to leave at the same time, seems to have rendered it desirable to increase the width of the channels.
In river-piers, and in landing-stages, the only considerations which affect the plan to be given to the structures are, the depth of the water, and the possibility of the transmission of the agitation from the open sea, or of the local agitation produced by the action of the wind upon a large surface of water. The latter phenomenon, as is well known, depends on the depth quite as much as on the extent of the water surface over which the wind blows ; and it is for this reason that, in broad deep rivers, such as the Southampton Water, the piers are made curvilinear in plan, in order to inclose and protect a species of port ; whilst on the shallow shores of the Thames, at such places as South end, a straight pier is found to be sufficient for the real necessities of the locality. The peculiar range of this tides at any particular place may render it necessary to vary the means adopted for facilitating the loading and unloading of ships; bUt a study of the various landing stages on the Thames will teach the student far more on this subject than It would be possible to do within the limits of this article.
2. As to the technical methods of constructing piers, they must evidently be regulated by the nature of the building materials fur nished by the locality ; and those structures may either be formed of wood, concrete, loose rubble stone, or solid masonry bedded in mortar ; they may be either perfectly closed on the water-line, or open, so as to allow any littoral current to sweep through them.
When wood is used, the foundations intended to receive the upper structure require to be executed with great care, and in such a manner as to allow the said upper structure to be repaired with ease, and with out affecting the foundations ; for it is essential to bear in mind, that the shock, or the abrasion, produced either by vessels, by waves, or currents, and that the ravages of the Teredo navalis, or of the Limnorict terebrans, are most injurious to the parts of a wooden pier placed between high and low water-mark. All those parts must, therefore,
be easily accessible, and renewable, either separately or entirely, as may be required. It Is customary to execute wooden piers with piles and whaling-pieces, fixed below the level of the ground of the fore shore, and upon these a series of trapezoidal frames, Carefully braced and bolted together, are placed. The lower part of these frames is covered with loose rubble, concrete, or fascines, in order to protect the foundations.
In open wood piers the frames are fixed at distances asunder of Irons 6 to 10 feet in the sea, according to the depth of water, and the habitual agitation of the latter at the place considered. The frames are made in the form of a trapezium, the inclined sides being respectively turned towards the open sea, or towards the sheltered channel, and forming an angle of from 13' to 33", with a vertical hue passing through their points of junction with the sill pieces. According to the width intended to be given to the roadway of the pier, the frames are made with one or more upright posts, between the inclined sides, and the whole are retained in their places by cross ties and raking braces converting the parts of the assemblage into a series of triangles, the sills and heads forming with the inclined sides the outer lines of the frames. Longitudinally these frames are connected by whales, stringers, and by the joists of the platform, and upon the latter the planking of the road-way of the pier is laid ; this planking is formed of rather narrow timber, about 4 or 5 inches thick, laid with intervals of about If or 1 inch. The planks are spiked down upon the joists, but upon their ends they are clipped by a bridging joist, bolted to the longitudinal sill fastened to the frames ; sometimes this bridging joist also bears a handrail of open timberwork. Tenon joints should be avoided in the carpenter's work of the frames, and under no circumstances whatever should the planking of the roadway be laid water-tight.