TIMBER DOCK. 



189 



to completely surround it. Between the outer sheet-piling and 

 the coping line of the dock there are four rows of 12-inch round 

 piles spaced 3 feet apart longitudinally, each row carries a trans- 

 verse sill or cap, upon which the wharfing is laid. 



The sides are further supported by diagonal braces fixed 

 to the upper side sills and the sloping side timbers. 



The dock entrance consists of heavy abutment and side 

 timbers, supported and backed by concrete and puddled clay. 

 The entrance is closed by an iron floating caisson. For making 

 a tight joint between the entrance stop and the keel and stems 

 of the caisson heavy indiarubber packing is used. 



FIG. 171. 



For clearing the water, drains are constructed in the concrete 

 under the floor timbers on each side of the keel-blocks, which 

 lead to the drainage culvert and pump well, which in this case 

 is near the dock entrance. 



For emptying the dock, there are two centrifugal pumps 

 of 42 inches in diameter driven by two vertical engines with 

 cylinders 28 inches diameter and 24-inch stroke. The pumps 

 have a capacity of 80,000 gallons per minute, and can empty 

 the dock in 00 minutes when no vessel is in it. The filling is 

 done through valves in the caisson. 



Another good example of this method of construction is the 

 dock completed in 1884, by Messrs. Simpson, at St. John's, 



