166 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



moderate-sized vessels for long periods before and after high 

 water, without unduly reducing the water in the basin ; this is 

 a matter of great importance where the water level has to be 

 kept up by pumping. This plan 1 of dividing a long lock is, 

 however, considered by some injudicious, inasmuch as it involves 

 the risk of breaking a long ship's back should it ground on the 

 middle sill. An alternative means of providing for the smaller 

 class of vessels, when it is necessary to pass them out singly, is 

 to have a short lock alongside the longer one. 



There being always a tendency to increase the draught of 

 vessels, it is a wise precaution to place the sill of a lock as low 

 as possible, although it may not be contemplated to make the 

 same depth in the approach in the first instance, and in muddy 

 rivers or estuaries there might be some trouble in keeping the 

 entrance clear, In after time, as the necessity arises, the depth 

 outside may be lowered with comparatively little difficulty ; but 

 to lower the sill of an entrance would be a very formidable and 

 costly operation. 



Whilst a batter on the face of an ordinary basin wall 

 is an advantage as adding to its stability, there is a decided 

 disadvantage, as regards the side walls of a lock, in giving 

 more than a nominal batter to the face ; even a nominal batter 

 is unnecessary, regarded as an aid to stability in view of 

 the support afforded to the walls by the lock floor. Although 

 with the lock full there may be ample room for two ordinary 

 vessels, yet, with much batter on the face, as the water level 

 is lowered, the effective width will decrease, and consequently 

 there will always be, when two ships are being passed out at 

 the same time on a falling tide, or at low water, some risk of 

 one being nipped against the side; the most satisfactory way 

 is to have the side walls of the lock vertical, so as to ensure the 

 same width at whatever the water level may be. Through the 

 entrance, a plain straight face is preferable, inasmuch as when 

 the gates are open there are no projections beyond the fair line 

 of the gates, and consequently no risk of the bow or bilge of 

 a vessel striking the quoin of the vertical recess provided for 

 the accommodation of the gates, which would be the case were 

 the side walls of an entrance constructed with a batter. Where 

 ordinary floating-ship caissons are used instead of gates for 

 closing the entrance, the side walls cannot be made perpendicular, 

 1 MI\LC.E,\o\. xcii. p. 177. 



