CALAIS HARBOUR. 



191 



accommodation for a vessel over 350 feet long being required, 

 a centre row of keel-blocks are provided. The entrance is 52 

 feet wide, with a depth of water over the sill and keel-blocks 

 of 25 feet 3 inches at high water of ordinary spring tides. In 

 addition to the outer gates, there are a set of inner gates, by 

 closing which a depth of 20 feet of water can be retained over 

 the keel-blocks. 



Calais Harbour. 1 Communication between the outer port 

 and the floating basin is obtained by two parallel docks of the 



SCALE 

 ip 20 



same length over all, but of unequal width and unequal effective 

 length between the gates (Fig. 174). 



The larger dock has a clear width of 68 feet 11 inches, and 

 the smaller one 45 feet 11 inches. 



The form and dimensions of the sections at different points 

 of the length are shown by Figs. 175 to 177. 



The level of the sills is so arranged that the depth of water 

 over them at high water springs varies from 22 feet 6 inches 

 to 28 feet 6 inches ; the extreme rise and fall of tide is 23 feet. 



Each dock is fitted with three sets of entrance gates, one 

 pair at the upper or basin entrance, and two pairs at the lower 

 or harbour entrance. In order to economize water in docking 

 vessels of different dimensions, each dock is also divided into 

 two chambers of unequal lengths by intermediate gates. 



In the wider dock, the total length between the sills of the 

 extreme gates is 505 feet 3 inches, and the clear length between 

 the inner or ebb gates 434 feet 3 inches ; when divided by 

 closing the intermediate gates, one chamber is 185 feet long, 

 and the other 249 feet 4 inches between the gates. In the 

 narrow dock, the total length between the sills of the extreme 

 gates is 521 feet 4 inches, and the maximum clear length be- 



1 Engineering, vol. xlvii. pp. 577, 611. 



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