4 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



lowering the water too much would be reduced to a very short 

 time before and after high water; the range of time would 

 depend, of course, upon the variation of water level in the dock, 

 which might be considered desirable, and which would be 

 governed in a great measure by the draught of the vessels 

 occupying the dock or basin. 



In order to meet this objection, an entrance lock is necessary, 

 and in some cases an outer or half-tide basin, by which adjuncts 

 the time during which vessels can be passed in or out can be 

 materially extended, and under certain conditions as to the water 

 supply, i.e. where the water-level in the dock is kept up by 

 pumping, entry and exit may be performed at any state of the 

 tide outside. 



Location and Class of Docks. With regard to the location and 

 class of docks, the following may be considered as some of the 

 principal conditions which govern the selection of the best 

 localities and most suitable class of docks. 



(a) Where the rise of tide is considerable, as, for instance, 

 on the west coast of England, where the rise of spring tides is 

 about 40 feet ; or the east coast of America, where the rise is 

 46 feet, closed docks or basins are more required than in rivers 

 or localities where the tidal rise is moderate, as on some parts of 

 the east coast of England, the south and east and north coasts 

 of Ireland, where ranges of 4, 6, and 7 feet are met with. 

 Where there is no tidal range, or practically none, as in the 

 Mediterranean, closed docks or basins may be considered un- 

 necessary, unless under exceptional local conditions. 



(b) 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 

 ships are of small size, and are therefore less likely to be strained 

 when not water-borne. 



(c) 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. 



(d) Where the harbour or anchorage is open to the entrance 

 of surface waves of considerable height, or of a ground swell, 

 there is more need for a dock than where there is better 

 protection. 



(e) Where there is sufficient fresh water, free from mud or 



