On Docks — Dry, Wet, and Floating. 201 



2nd. Those where the receptacle for the ships is itself 

 afloat, heing, in fact, a floating chamher, large enough to 

 receive a vessel, with means and appHances for preventing 

 the ingress of water, and of getting rid of it should any 

 leakage take place. 



There is also a third class, which, however, can hardly be 

 called a dock, inasmuch as when the vessel is floated into its 

 place, it is afterwards lifted bodily out of the water by means 

 of some mechanical agents. 



In considering the construction of those belonging to the 

 first class, one of the first circumstances to be taken into 

 consideration is the rise and fall of the tide, at the point 

 where the dock is to be made. Should the rise of tide be 

 great, (exceeding, in fact, the draught of water of the vessels 

 for whose use the dock is intended), the difficulty of the 

 construction and working of the dock is very much diminished 

 — the foundation being above low water-mark, the whole of 

 the work can be done without incurring the expense of 

 constructing a coffer-dam to keep out the water. As the 

 water ebbs below the floor of the dock, no provision need be 

 made for pumping the water out of it, as it can be allowed 

 to flow out at ebb tide. 



Little or no difficulty will be experienced in keeping the 

 apron in front of the entrance clear for the opening of the 

 gates, as it will be exposed at every tide. 



The construction, therefore, of a dock in a tidal river, 

 when the rise of the tide exceeds say 20 feet, is the simplest 

 operation of the kind ; and I will proceed to give an account 

 of the difficulties which are likely to be met with, and the 

 precautions which must be taken, in order to secure the 

 work as it proceeds. 



The first matter to be looked to is the foundation of the 

 proposed dock, — Is it solid enough to sustain the load which' 

 it will have to carry without the assistance of piles, or some 



