TIMBER DOCKS. 187 



as follows : The original cost is comparatively low, being from 

 30 to 50 per cent, less than the cost of masonry. 



Docks constructed of timber resist the action of frost better 

 than those constructed of masonry. The alternations of excessive 

 heat and cold peculiar to the North-American climate set up 

 extreme expansion and contraction, causing the joints to 

 separate and the water to enter ; hence, through the severe 

 frosts of successive winters, the stones are apt to be dislodged. 



The cost of repairs and maintenance is stated to be less than 

 in the case of masonry under like conditions. Experience of 

 some of the oldest timber-constructed docks in America shows 

 that extensive repairs are required only after twenty years, and 

 then only to the most exposed parts, such as the floor planking 

 and altars; whilst the piles, keel and floor timbers, sill, and 

 abutments remain in a good state of preservation. 



Wood is safer than stone to work upon in severe weather, 

 inasmuch as ice is not so liable to form and remain on wood as 

 on stone. 



The low and narrow altars afford a safe and easy means of 

 access to the bottom of the dock, and as they entirely surround 

 the dock, no choice of a place is needed in going to and from 

 work. Moreover, the low angle of the sides admits of more light 

 and air to the ship's bottom, and, consequently, the discomfort of 

 the damp, chilling air inseparable from the use of stone docks in 

 winter is largely obviated. 



The method of construction adopted by Messrs. Simpson and 

 Company, of New York, is shown by Figs. 171, 172, which 

 represent the half -trans verse section of a dock of this type lately 

 completed at the Brooklyn Navy Yard, United States. 



The dimensions of this dock are 



Ft^ in. 



Length over all on coping ... 530 



inside of caisson 500 



Width on top 'midships 130 4 



floor 'midships ... 50 



floor at entrance , ... 53 



Width of entrance at coping 85 



Depth of sill below coping 30 6 



Depth to floor of dock below coping 32 8 



Depth of water over sill at high tide 25 6 



The entire area of the floor is enclosed with continuous 8-inch 

 grooved-and-tongued sheet-piling, the practice being in good 

 ground to drive this sheeting to a depth of 7 feet 6 inches. In 



