368 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



port of Dunkirk, made the following comparison of the cost of 

 wood and iron gates, enclosing respectively two locks of the 

 same width of 69 feet, and having the same rise of one-sixth 

 the span. The outer faces of the gates were flat ; the wooden 

 ones were 23 feet 6 inches high, and 3 feet thick in the middle 

 of each leaf ; whereas the iron ones were 27 feet high, and 

 3 feet 7 inches thick in the middle. The lower part of the 

 iron gates, down to the mean sea-level, formed a closed 

 chamber, one part of which afforded flotation, and the other 

 contained water-ballast. Both pairs of gates could safely bear 

 a load of water corresponding to an equal difference of level 

 of 22 feet between the dock and the sea. In both, the material 

 had been distributed as economically as possible. These con- 

 ditions afforded a favourable basis for comparison. 



The price of equal surfaces was the same for both gates 

 if the iron were galvanized, but the non-galvanized iron gate 

 cost only four-fifths the price of the wooden one. The chances of 

 durability were at least as great for the gate of galvanized 

 iron as for that of wood, and they were almost as great for the 

 non-galvanized iron where the paint could be often renewed. 



Wood gates. 



Iron gates. 



Surface under pressure for 



one leaf 

 Total weight of one leaf in air 



Weight of wood and iron per 

 square foot of surface under 

 pressure 



Effective weight of one leaf 



immersed at mean sea-level 



Effective weight of one leaf 



completely immersed athigh 



water 



In 1856, date of con- 

 struction of wooden 

 gates. 



In 1879, date of con- 

 tract for iron gates. 



Total 

 cost 



of one 

 leaf. 



Price per square foot of sur- 

 face under pressure in 1879 



877 square feet 

 50 tons 

 126 Ibs. 



32 tons 

 10 tons 



9GO 



20 per cent, more 

 than in 185C, or 

 1 15, 



1 6. 



984 square feet 



Iron and wood, 49 tons ; water- 

 ballast, 8 tons; total, 57 tons 

 112 Ibs. 



16 tons, including water- 

 ballast 



10 tons, including water- 

 ballast 



Iron, not galvanized, but 

 scraped, passed through 

 boiling oil, and painted with 

 four coats of paint, 1000 



Iron, galvanized and pain tod 

 with four coats of paint, 

 1300 



Iron, not galvanized, but 

 passed through boiling oil, 

 and painted with four coats 

 of paint, 1 1. 



Iron, galvanized and painted 

 with four coats of paint, 

 1 G. 



