220 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



without deterioration or decay. That, when constructed of this 

 material, gates are quite as cheap, if not cheaper, than those con- 

 structed of iron ; and that they are equally as durable. Also that 

 wood, owing probably to its more yielding nature than iron, per- 

 mits the gate to accommodate itself more readily to the sill, and 

 where bearing-rollers are employed, to adjust itself better to any 

 slight irregularity in the roller path ; further, that gates properly 

 constructed of timber will, on the whole, stand a greater amount 

 of rough usage without serious results as compared with iron. 



The principal disadvantages of timber are that, unless 

 efficiently protected, the attacks of the marine worm will do 

 more harm in a few months than natural deterioration and 

 decay will do in years ; and for some forms, as, for instance, 

 circular gates for large spans, the difficulty of obtaining logs of 

 sufficiently large scantling to serve for the continuous horizontal 

 beams. To meet this difficulty, however, it is the practice to 

 construct each leaf in panels or divisions, which are bolted 

 together at the intermediate vertical timbers, and by connecting 

 pieces bolted to the horizontal timbers. With this sectional 

 principle of construction, the timbers used need not be of 

 unusual dimensions. The gates can be built in a workshop, 

 and then readily removed in parts to their place for erection. 

 If properly constructed, they will stand a considerable amount 

 of rough usage. They require, however, to be well housed in 

 the recesses, and guarded against heavy blows from passing 

 vessels; such blows will not as a rule injure gates with con- 

 tinuous beams, whether of wood or iron ; but there arises the 

 risk of dislocation from such blows acting on the concave side 

 of timber gates constructed on the sectional principle, which 

 does not admit of great resistance to a strain contrary to the 

 usual direction of the water pressure. 



Metal Gates. There was and still is with some engin 

 great prejudice against the use of iron or steel as a material of 

 construction ; the objections advanced being that gates so con- 

 structed are difficult to keep water-tight, that they are much 

 more liable to damage by vessels striking against them th.m 

 timber gates, and that metal is subject to much more rapid 

 deterioration from rust and the action of sea water than timber. 

 Practically, there appears to be no valid reason for this prejudice, 



The advantages 1 of iron and steel arc durability, strength, 

 1 A1.PJ.CK, vol. lv. p. 113. 



