TIDAL HARBOURS. The enclosures for the protection of vessels which are situated upon the sea shore, or the portions of rivers affected by the rise of the tides, are usually known by the distinguishing name of tidal harbours, on account of the peculiar arrangements adopted in them in consequence of the variations of level in the waters, and of the currents which may prevail in the offing. Tidal harbours may bo floating or dry, according to the rise of the tide, or the depth of water, when no artificial basins are formed ; or they may be classified as natural or artificial harbours, according to the configuration of the coast, and the mode of construction adopted in them.
A natural floating harbour Is one wherein there is at all times of the tide a sufficient depth of water to maintain a vessel afloat ; a natural dry harbour is one which is left without water at low tides, and in which the vessels are obliged "to beach," or " to take the ground," that is to say, are left temporarily high and dry. Artificial harbours present occasionally one, or both, of these conditions ; but as they are usually formed in positions where the navigation is sufficiently active to require great facilities for the commerce carried on in them, it is rare that artificial tidal harbours are constructed without the forma tion of floating basins in which large vessels may be kept constantly afloat. Dry tidal harbours are only of use for coasting traffic, or for vessels of small burden. The outer harbours of ports of greater importance are often in fact nothing but tidal harbours, dry at low tides ; but in such cases they are accompanied by half-tide basins, and inner harbours or docks, in which the level of the water is constantly maintained at that of the high tide by means of lock gates, or pontoons. Graving docks, gridirons, scouring sluices, warehouses, and other appliances are added in such cases according to the local physical conditions of the situation, to the nature of the trade, or to the fiscal organisation of the country in which the harbours may be situated. These details have already been discussed under DOOKS • and they evidently must be the same in all import harbours, whether tidal, or otherwise; or whether the outer harbour be wet or dry.
Upon an open coast, like that of the southern counties of England, excepting between the Race of l'ortland and Selsea Point, the harbours must all be tidal, and dry at low water, unless they should be placed at the mouths of rivers, or at the head of deep inlets from the sea.
Milford Haven and Plymouth are instances of this favourable natural configuration, and there is little necessity for the formation of closed docks in either of them. Southampton and Portsmouth present very great advantages also, and they are almost entirely devoid of the inconveniences usually attached to tidal harbours, for vessels can lie in the inclosed entrances to the floating basins of either of those ports without " touching," unless when their burden is very great indeed.
In the case of ordinary tidal harbours, it therefore follows that, unless there should exist near them a good sheltered roadstead, vessels cannot safely " make " them, unless at high water ; nor can they ever become really "harbours of refuge," which should be accessible at all times of the tide. Havre and Liverpool are instances of tidal harbours pre senting tolerably favourable conditions of access in consequence of the existence of roadsteads. Cherbourg is an instance of a tidal harbour rendered safe by the creation of an artificial roadstead ; for though the military port of Cherbourg has a sufficient depth of water to allow the largest vessels to enter at any time of the tide, yet the commercial port is left high and dry twice a day. Ramsgate, Dieppe, and the little harbour of refuge (so called) at Port en Bessin are instances of the true tidal harbours,with all their characteristics and all their inconveniences. They can only be entered at high tide ; their jetties are so much exposed, that if a vessel should happen to miss the entrance she would almost infallibly be wrecked ; and they are all liable to be silted up by the alluvial matters carried forward by the flood-tides on their shores. [TIDAL WATERS AND CURRENTS.] Under PrEsts, the modes of constructing and the principles of designing the walls which inclose the spaces intended to form harbours have been already discussed ; and it may, under these circumstances, suffice to say that the area to be inclosed in a tidal harbour designed to receive ordinary coasting vessels should not be less than six acres of water surface ; and that the area should increase from this minimum dimension in proportion to the depth of water and to the activity of the commercial relations of the locality. In the case of tidal harbours of refuge, the area must be proportioned to the number of vessels likely to resort to them. Thus the tidal harbour of Southampton Docks, in which there is 18 feet of water at low tides, has an area of 16 acres ; the area of the tidal basin of the new port of St. Nazaire, at the mouth of the Loire, is about 225 scree ; that of the tidal harbour of Port en Bearin, is about 27 acres ; that of the outer harbour of Ramsgate is about 305 acres. Even in dry tidal harbours it is desirable that the extremities of the jetties should be carried out into 6 feet water at low neap tides ; for the minimum depth of water at the entrance should never be less than 18 or 20 feet.