On Docks — Dry, Wet, and Floailncj. 203 



inverted arch, and taking particular care that the joints of 

 the masonry are tight. 



When the floor is of wood, the beams of which it is made 

 should be near enough together and strong enough to with- 

 stand the pressure, and the planks spiked upon them should 

 be carefully caulked. 



The weight, too, of the walls should be calculated to 

 withstand the maximum lift which might be exercised by 

 the upward pressure of the water, as well as the pressure of 

 the earth behind them. 



The floor and the walls being constructed with all the 

 arrangements which may be desirable, in order to facilitate 

 the work of the sliipwrights, — such as steps from bottom 

 to top, slides for timber, altars or steps for the shoring 

 timber to rest against, — the next thing to consider is the mode 

 in which the entrance should be closed. This may be done 

 in two ways, — either by gates, or by a caisson, which is a 

 floating vessel made to fit into grooves in the sides and 

 bottom of the dock entrance, and which can be floated into 

 its place and then sunk so as efi'ectually to close the opening. 



The question as to which of the two plans is to be adopted 

 turns upon a variety of considerations. When the opening 

 of the dock is not very wide, and there are means of clearing 

 the apron in front from any collection of mud or sand, 

 gates would probably be the cheapest, and most certainly the 

 easiest mode of closing the entrance ; but when the opening 

 is very wide, as in the case in docks which are required to 

 admit steamers of a large class, and where there is the risk 

 of collecting mud and sand to any great extent upon the 

 aprons in front of the gate, which it would be difficult to 

 remove on account of the depth of water over the sill at 

 low water, then a caisson is the cheapest and most efiectual 

 mode of keeping out the water. There is an advantage 

 attendant upon a caisson even where the water does ebb 



Q 



