24 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



(d) A tidal basin acts as a reservoir from which issues a 

 great volume of water every ebbing tide, which assists in 

 scouring the entrance. 



(e) The first cost of a single pair of gates, or of a caisson 

 with relative masonry, or of a double pair of gates, or two 

 caissons with a lock, together with the annual cost of working, 

 is saved. 



Tidal Dock at Southampton. 1 This dock, constructed from the 

 designs of Mr. A. Giles, M.I.C.E., is on the foreshore to the west 

 of the old Southampton Docks, and encloses a deep-water area of 

 18 acres, with a total length of quay wall of 3854 feet run. 



FIG. 15. 



The plan of this dock (Fig. 15), of a diamond shape, is some- 

 what novel, but it is perhaps the best that could have been 

 adopted to meet the peculiarities of the site, and in view of the 

 best position for the entrance, which is placed at the south 

 angle, and has a direct lead by a deep-water channel to 

 Southampton Water. 



The bottom of the dock is 45 feet below coping. The level of 

 the water at high springs is 6 feet below coping, and at low 

 springs 19 feet below coping, or a rise and fall of 13 feet ; the depth 

 of water in the dock and its approaches is therefore never less 



1 Engineering, vol. 1. p. 308. 



