On Docks — Dry, Wet, and Floating. 205 



but, as in this case, tlie whole space intended to be occupied 

 by the dock must be excavated to the necessary depth below 

 the surface of the water in the river and harbour, he wilL 

 in addition, have to exercise every precaution to prevent the 

 water of the river making its way into the excavation. A 

 coffer dam, as it is called, must be constructed, steam-engines 

 and pumps provided, to keep the work clear of water ; and 

 after every precaution has been taken it may be found that, 

 owing to the porous nature of the soil, or other circumstances 

 connected with its geological character, it may be imprac- 

 ticable to execute the work except by means of a diving 

 bell, — and that at a cost altogether incommensurate with 

 the object to be attained. 



At Bermuda, the Ireland Island, where the Government 

 dockyard is placed, is composed of a porous limestone, an 

 aggregation of broken shells and coral sand, the whole being 

 so pervious to water that an excavation at any point carried 

 a foot or two below the level of the sea is filled with salt 

 water in a very short time. By no means, then, but by the 

 diving bell could either the necessary excavations be made, 

 or the masonry of the floors and of the sides of the dock 

 be constructed in such a situation. 



A coffer dam is not merely an expensive undertaking, but 



it requires constant care and attention : it is generally made 



of two rows of sheet piling, about 5 feet apart — the 



interval is filled with clay puddle ; care must be taken to 



connect the two rows of piles together by means of strong 



bolts to as great a depth as the water will allow, — the pressure 



against the piles by the weight of the clay and its tendency 



to expand by wet being very great. I have seen a pile, 12 



inches square, broken short off by the pressure of about 20 



feet of wet clay, the weight of which would not have exceeded 



130 pounds to the foot : the effect, therefore, could not be 



attributed to the weight of the clay, but to the expansion 



caused by its absorption of water. 



q2 



