204 On Docks — Drij, Wet, and Floating. 



nearly away from the apron, inasmuch as it is possible to retain 

 a head of M^ater in the dock, and to let it out by means of 

 sluices through the caisson, — thus creating a scour in front 

 of the dock entrance, which may keep it clear of deposit. 



When I stated that, under certain circumstances, gates 

 would prove the ' cheapest and easiest mode of closing the 

 entrance, I referred principally to those which, turning upon 

 heel posts fitting in a groove in the piers of the dock, meet 

 at an angle, and are supported against the pressure of the 

 water, — partly by the sill against which each leaf rests at the 

 bottom, and partly by each other : the angle at which the 

 leaves of the gate meet render it impossible that they should 

 yield to the water, unless the piers against which the pressure 

 is thrown should give way, or the framing of the gate itself 

 prove too weak. There is, however, a simple kind of gate, 

 composed of three portions, two of which rest in grooves, 

 or against projections in the sides of the piers and against 

 a sill at the bottom ; wliile the third fits in a sort of keystone 

 between them. These are allowed to float away when not 

 required, or to lie on the apron, — are hauled into their 

 proper position by ropes, — and, when once fixed, have the 

 additional support of shores against the piers, as shown in 

 the sketch. This arrangement is cheap in its original con- 

 struction ; but it is clumsy, and requires a good number of 

 hands to work it properly. The objection to the caisson 

 is, that it requires a number of hands to work it, especially 

 if the tide does not ebb sufficiently to allow of the water 

 which sinks it into its place being drawn oiF, the labour 

 of pumping water out of a caisson is very great. 



These are the principal matters which have to be attended 

 to when a dock has to be constructed in a river where the 

 tide ebbs away below the level of the floor. The same diffi- 

 culties will have to be surmounted by the engineer in 

 constructing a dock where there is httle or no rise of tide : 



