Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/494

Rh 466 entered for the purpose of being repaired. The water below low tide level is pumped out generally by steam power. The wet dock or tide basin is a large water-tight in- closure, usually several acres in extent, which is also fitted with gates which are closed, not, however, during the flowing but the ebbing tide, so that the vessels lying at the quays within the basin may remain constantly afloat and nearly at the same level while being loaded and unloaded. In order to extend the time during which vessels can enter or leave a wet dock there are two additional works which are often connected with it. These are the entrance- lock and the outer or half-tide basin. The lock is a narrow chamber of just sufficient length and breadth to admit a ship of the greatest length and breadth of beam that is expected to frequent the port. Such a lock can, in absence of the largest class of ships, accommodate simultaneously three or four vessels of smaller tonnage. Locks are some time made with double gates, so that when only one vessel of small size has to enter, she isshut up within a compartment of the lock, by which means some of the water is saved which would have been expended had the whole of the chamber been filled. Shortly after the tide has begun to ebb, and when the gates of the wet dock have been shut, in order to prevent the water from falling too low in the dock, a vessel can still enter or leave by means of the lock. She is first passed into the lock from the sea ; and, the lower or sea gates being closed, the sluices in the upper or dock gates are opened, which raises the water in the lock chamber to the same level as the water in the dock, when the upper gates are opened to let the vessel pass in. Ships leaving the dock after the tide has begun to fall are passed into the lock chamber, when the upper gates are closed and the sluices in the lower gates are opened, until the water in the lock has fallen to the same level as the tide outside, when the lower gates are opened and the vessel goes out to sea. This system of locking is continued so long as the level of the dock is not so much depressed as to affect the loading or unloading of vessels lying at the quays, or until the tide has ebbed so far as to prevent vessels reaching the lock from the sea. At the West Dock, Cardiff, where the level is kept up by fresh water from the river Taff, the gates, in order to suit the rise and fall of the tide, are opened one hour before and about two hours after high water. The tide basin may be described as a lock of very large dimensions capable of receiving at the same time a con siderable number of vessels, which continue to run into it from the sea after the dock gates have been finally closed for that tide. Such vessels can run in, so long as they have sufficient water over the sill of the sea gates of the basin, or until the basin itself is filled with ships. The sea gates are then closed, and the sluices in the upper gates are opened till the water in the basin is on a level with the water in the dock. The ships are then passed continuously into the dock, in a string, one after the other. By means of this arrangement the traffic can be carried on to a far greater extent than is possible with a narrow lock, which has to be filled and emptied for each vessel that enters or leaves the harbour. The late Mr Jesse Hartley of Liverpool was the first to propose half-tide outer basins, as adjuncts to wet docks. The peculiar advantages .of docks are the following. Vessels can be accommodated in the smallest possible space, anf j are ena ki ec i t n e constantly afloat ; whereas in tidal harbou-rs, where they take the ground, they are apt to be strained or to have their floors broken. But there are other sources of mischief than this in open harbours, for often, when the tide is ebbing, vessels, unless watched, fall [DOCKS. against each other. In two instances where the bottom, which was muddy, had a considerable declivity, a ship which had taken the ground on the beach near low water mark was actually run down and damaged by another stranded vessel, the warps of which suddenly snapped and freed her from the moorings at the quay, thus causing a collision between vessels both of which were at the time high and dry. Then there is the chafing of the vessel s sides against the quays in exposed harbours where there is a run of sea, and the breaking of warps during stormy weather, or during land floods, where there is a river. The late Mr James Walker stated that at Sunderlanrl damage to the extent of 40,000 was occasioned in a single day by large quantities of ice that came down the river Wear. When a vessel is in dock she can be easily and at all times moved from place to place, and the operation of discharging and loading can go regularly on during all times of tide. Her level, too, is never much affected, so that the cargo does not require, when being discharged, to be hoisted so high as would otherwise be necessary. Timber vessels can be unloaded through their ports into the water at all times of tide, while in harbours which are dry at low water the cargo receives injury by falling on the hard bottom. From the general description which we have given the following conclusions may be drawn as to the best localities for docks : (1) Where the rise of tide is considerable, a dock is Best more required than where the rise is small. In rivers such lo cali as the Clyde and the Foyle, for example, where the tide *. 01 d _ ; rises only from 9 to 10 feet, the inducement to construct docks is not so great as at the Mersey or the Bristol Channel, where the tide rises from 20 to 50 feet. (2) Where the nature of the trade requires ships of great length, which are therefore very liable to injury from taking the ground, a dock is more needed than at places where the vessels are of smaller size, and therefore less liable to be strained when not water borne. (3) Where the bottom is soft and muddy, there is less need for a dock than where the bottom is hard and uneven, unless where strong currents are likely to burrow out the soft soil so as to make the bottom irregular by forming deep ruts. (4) Where the harbour is open to the entrance of sur face waves of considerable height, or of a ground swell, there is more need for a dock than where there is better protection. (5) Where there is sufficient fresh water, free from mud or silt, for supplying the basin, a dock will be more suitable than where the supply is from the tide, especially if the sea at the place be much loaded with matter in mechanical suspension. In designing a dock the first subject to which the engineer Sea c must give his attention is the seaward exposure of his pro- posui posed work. Of course no precaution is needed on this head if the place be a land-locked bay, or a narrow river or estuary. But if there be a considerable fetch of open sea, the waves generated during gales may be of such height as to endanger the sea gates or to interfere with their being worked in stormy weather. The formula already given at page 457 for short lengths of fetch will be the most suitable for calculating the exposure of docks, where the fetch ought never to be great. Having calculated by means cf this formula the height of waves that maybe expected to reach the dock gates, tho engineer must then determine whether such waves are too high for the strength of the gates, or for their being properly worked in stormy weather. Now we have unfortunately but few facts on this subject to guide us, and these not very consistent with each other.