ENCLOSING DAMS. 



73 



upper end of the inclined shore abutted against the piles and 

 under the upper waling, whilst the lower end rested upon the 

 horizontal shore and against the shoring-piles. 



The dam piles were secured to the back shoring-piles, and the 

 horizontal shoring-piles kept in place by long tie-bolts placed 

 immediately over the horizontal shores, and screwed up on cast- 

 iron washers, 13 inches by 9 inches, webbed at the back. 



This dam was also very pliant, and yielded considerably to 

 the varying pressure of the tides, as had been anticipated, but 

 no ill consequences resulted. 



Hard Clay 



SCALE. 



50 FEET. 



FIG. 51. 



A Circular Dam 1 (Figs. 52, 53), constructed in the steam 

 basin of the dockyard, was designed to maintain the water, 

 in that basin, so as not to interrupt its use while one of its 

 walls was cut away, and a new entrance built to connect the 

 tidal basin with the steam basin. 



The dam was constructed to the segment of a circle of 162- 

 feet radius to its outer fan ; it consisted of two rows of whole- 

 timber piles driven as firmly into a strong clay as the timber 

 would endure, or until they met with septaria which the clay 

 contained. They were 5 feet apart in the clear, and were very 

 strongly supported by laminated walings of whole timbers 

 cut to the proper curve, the ends breaking joint, a space being 



1 M.P.I.C.E.. vol. Ixiv. p. 138 



