4fi 



assumes an intermittent and tedious character. The difficulties 

 attending unfavourable strata in the foundations are readily over- 

 come by driving piles or sinking concrete cylinders. The latter have 

 ■ been used very successfully on the Clyde for forming the sub- 

 structure of quay walls reaching 20 feet below low water mark. 



Where space will permit, it is usual to form a Lock at the entrance 

 to a dock with a double pair of gates, the distance between which is 

 regulated by the class of vessels frequenting the port. At high water the 

 inner gates are closed, so as to maintain the water level in the dock, and 

 thus prevent the possibility of the larger class of vessels within taking the 

 ground. The outer gates may be kept open so long as there is water 

 enough in the harbour to admit a vessel. If one should arrive and wish 

 to enter the dock, she is moored in the lock, the outer gates of which are 

 then closed. The water level in the lock is then raised to that in the 

 dock by means of sluices through the inner gates or side walls ; when this 

 is done, the inner gates are opened, and the vessel allowed to enter the dock. 

 The reverse operation of passing a vessel out is performed in a similar man- 

 ner. In some cases an outer, or what is termed a " Half-tide Dock" is 

 provided, capable of holding several large vessels and which, in 

 addition, is used as a dock proper by the smaller class of vessels. 

 By fixing a pair of gates at the entrance to Whitehaven North 

 Harbour it could be utilised as a half-tide dock and kept open for ship- 

 ping an hour after the gates of the inner dock were closed ; during which 

 time a spring tide would have ebbed about 2 feet 4 inches ; this would 

 represent the difference in level of the water in the inner and outer docks. 

 But as they are about of equal area, by opening the inner gates to allow 

 vessels to pass in, the general water level would be reduced only one-half 

 the original difference in level, or 1 foot 2 inches. 



In a busy harbour the advantage of a lock or half-tide dock is mani- 

 fest, but in an important seaport with a long chain of docks it is almost 

 indispensable. Vessels arriving late during the tide, having been delayed 

 by fog or stress of weather, are passed through to their appropriated berthg 

 and enabled to discharge their cargoes of perishable goods and leave by 

 the next tide. 



