206 On Ducks— Dry, Wet, and Floating. 



Great caro must be taken to support the coffer dam against 

 the pressure of the water by a proper system of shores ; and 

 this it may be imagined is not a matter of trifling importance, 

 when it is considered that a head of 20 feet of water 

 outside a coffer dam gives a pressure upon every foot of its 

 length of Sg- tons ; and this, if the bottom of the pile be 

 considered as fixed, is acting with a leverage of 10 feet to 

 turn it over. 



It is usual to place the shores at a distance of about 10 

 feet apart, in a horizontal direction, and of from 6 or 8 to 

 4 feet apart in a vertical direction ; and even when placed at 

 such short distances, the strain thrown upon them is enor- 

 mous, calling for every possible precaution and attention on 

 the part of the engineer. 



While the coffer dam is in course of construction, care 

 must be taken that the water inside is never allowed to 

 remain at a higher level than that outside, otherwise the 

 whole fabric will be forced outwards. Sluices, therefore, 

 must be provided, by which a free communication may be 

 established between the interior and exterior. 



When the coffer dam is completed and the ground pre- 

 pared, the work will proceed in the same manner as before 

 described. All the precautions which were required in the 

 dock built in the tidal river will be required : indeed, greater 

 care should be taken, because, should any accident happen, 

 it can only be remedied by the adoption of expensive means 

 of shutting out the water and pumping the dock dry ; while, 

 in the other case, the repairs can be executed at once. 



Having now said all that is necessary upon the subject of 

 the first kind of dock, I will proceed to give a short sketch 

 of the modes in which the floating docks, or those lifting 

 docks mentioned under the third head, are brought into 

 operation, merely as an introduction to the descriptions of 

 those American inventions, drawings of which I now lay 

 before the Society. 



