It may, perhaps, be advisable to state that tidal harbours whose entrances are exposed to be swept by a strong flood current should, generally speaking, have the jetty against which the flood strikes carried beyond the jetty on the oppoaiteside ; whereas in deep-water harbours the relative lengths of the jetties are regulated by their positions with respect to the direction of the prevailing wind. In fact the extension of the jetty on the outside of the flood may give rise to a local counter-current which would facilitate the entry into the har bour ; and in some respects it may even cause the tide to " stale," as seamen say, or to remain for a short time at a constant height in the inclosed space, in consequence of the resistance offered by the great ebb-tide in the offing to the efflux of the small quantity of water in the port. The various currents which prevail in the British Channel, at the mouth of the Seine, and of the Southampton Water, may be referred to as illustrations of these peculiar conditions ; and at Havre they occur in such a manner as to render the tidal outer harbour of nearly as great value as an ordinary floating harbour during two hours of each high tide, whilst at the same time they create a strong current setting into the tidal harbour during the flood, or precisely at the period when vessels are entering. Sometimes, however, these local currents give rise to bars or sand-banks, either across the mouth of the harbours or on the down aide (to the flood) of the main channel, in consequence of the interferences they produce with the advance of the alluvions carried forward by the flood. Thus, at Dieppe, Boulogne, Newhaven, -Harwich, Harlingen, &c., bars exist at the mouths of the tidal harbours, and they are usually of a very dangerous character, although occasionally, as at the last-named harbour in the Zuyder Zee, the bars may form natural breakwaters, inclosing shallow and imperfect roldsteade. There are, In fact, very few positions in which tidal har bours are free from inconvenience, arising either from bars or from the advance of alluvial matters. The tidal harbour of refuge of Port en Bessin, in the department of the Calvados, presents in this respect some peculiarities to which it may bo desirable to call attention, on account of the geological interest, quite as much as on account of the lessons in engineering, which they furnish. Port en Basin is erected on a coast entirely open to the north, north-east, and north-west; the most dangerous storms blow from the north-east ; and the flood-title comes in from west by north. The jetty on the west ilia has been built with openings, for the avowed object of allowing the flood to sweep through the head of the harbour ; and an opening is left in the eastern jetty to allow a fresh-water river, which rises from between the clay beds of the lower oolitic formations on the shore, to escape. This river would
appear to be the continuation of the little river Drome, which loses itself about two miles in the interior, there passing between the forma tion known locally as the "argils du Port en Basin " and the lower Naito Itself, until it thus escapes on the aea-shoro.
In consequence of the frequent occurrence of bars and banks at the months of tidal harbours, it is more than usually necessary that tidal signals. lighthouses, and fog-bella should be established upon them, and that the navigable channel should ho carefully buoyed. These precautionary details must not, of eourso, be neglected in floating tidal harbours, if the latter should present any local peculiarities ; but they are usually less necessary than in dry harbours; nor does there exist in the former the same necessity for the existence of a soft bottom, of mud or of sand, that exists in harbours whereiu vessels aro likely to take the ground. In floating or in ordinary tidal harbours which have docks for the reception of large vowels, the entrances to the docks must be placed at a position in the outer basin removed from the agitation of the open sea ; and it would appear from the practical working of the docks at 1 f Avre, Liverpool, fie., that it is preferable to make the entrance to the floating dock from an exterior half-tide dock, rather than from the outer harbour itself. There is, in fact, a danger of the gates communicating with the outer harbour being occasionally forced open by the agitation of the latter ; and at all times the half tide basins facilitate greatly the manatuiress of a port. In some tidal harbour., as at Havre, Dieppe, Ostend, &o., paine are taken to break the waves which may be driven in from the open sea, by the erection of timber stockades and inclined floors of masonry; but in the majority of eases it is found that the waves become sufficiently stilled by the fact of their passing through a narrow passage at the entrance, and then widening out into a large sheltered area.
(Sir J. Rennie, on harbours; Sganzin, Coors de Construction; 31inard, Trorass, hydrouliques se la the Parliamentary Reports on the highland floods, &c. ; Smeaton's Reports ; Life of Telford ; &c., Sc The Report of the Coremiesion on Tidal harbours, 1845, may also be consulted.)