202 Oh Docks— Dry, Wet, and Floating. 



other artificial support ? If solid enough for that purpose, 

 is it pervious to water ? On the answer to these questions 

 will depend, in great measure, the cost of the construction. 



Should the foundation be, as is often the case in tidal 

 rivers, a mere mud- bank, it will be necessary to provide a 

 solid bearing for the floor of the dock and the side walls, by 

 driving such a number of piles as may be necessary to 

 sustain the whole weight of the dock and of the vessel 

 which may be under repair in it. Upon the piles will the 

 floor of the dock be laid. If the material of which it is 

 constructed be stone or brick, a platform of timber will be 

 required upon the top of the piles. Should the ground, 

 however, be solid, it will be levelled to receive the floor of 

 the dock, which can be built at once upon it. 



Whether, however, the ground be hard or soft, — whether 

 it be necessary to use piles or not, there is one most essential 

 precaution to be taken in laying the floor ; for want of 

 proper attention to which many accidents and failures have 

 taken place. 



The floor, however constructed, should be able to resist 

 the upward pressure of the water. At high water, when the 

 dock is completed, the gates closed, and the docks empty, 

 should there be any means by which the tidal water could 

 make its way under the floor of the dock, a thing not 

 unlikely to happen, there would then be an upward pressure 

 equivalent in pounds weight to the area of the part thus 

 acted upon by the water, multiplied by the height of the 

 water outside above the floor, and this multiplied again by 

 G2^ lbs., the weight of a cubic foot of water. 



If we suppose the area of the floor to be 120x30=3000 

 feet, and the height of the water 20 feet, 3600x20x62^= 

 2009 tons. 



Provision is made against this pressure when the floor is 

 constructed of stone, by making it in the shape of an 



