A Treatise On The Principles And Practice Of Dock Engineering

STRENGTH. 3°9 the vulnerability of timber gates, yet it may be claimed that the injury was far from vital, that the repairs were speedily effected, and that in undergoing a similar experience, the damage to a pair of iron gates would have been well-nigh irreparable. The veriest trifle, indeed, may cause them serions if not fatal injury, owing to the thinness of their skins, the rigidity ■of their rivetted joints, and the delicate adjustment of their buoyancy •chambers. Several instances might be cited, but the following extract,* relating to a pair of iron gates at Limerick, will suffice :— “About 1867, the bottom plates were unaccountably injured. The air-cells filled with water, which it was found impossible to eject, as no provision had been left for pumping. The result was a total loss of buoyancy, the whole weight of the gates being thrown on the bottom pintles and rollers. Temporary repairs to the damaged plates were effected by divers, and sluice doors were placed over the inlets on the river face, so that the effect contemplated by the designer was reversed, the air-cells and water-cells changing their functions. This arrangement was partially successful, but had the disadvantage of imparting such an excess of buoyancy to the gates that during rough weather, at spring tides, they were nearly floated off the hinges, whilst at neaps as many as twelve men were often required to move them. The state of things grew worse, for the roller carriages became disabled under the undue stress, causing the gates frequently to jamb in the closing, allowing the water to leave the dock.” After this, it is not surprising to learn that the ■estimated cost of repairs rendered an entirely new pair of gates advisable. After receiving a number of reports on the relative merits of wood and iron gates, followed by a general discussion, the Ninth International Navigation Congress, sitting at Düsseldorf in 1902, came to the conclusion that no definite opinion could be expressed as to the preference to be accorded to wood or iron gates, the question depending almost entirely upon local considérations. They adopted a further conclusion, however, “ that for locks of great width, iron gates offer the advantage over wooden gates that they can be more easily constructed with suitable stiffness and durability, more readily and expeditiously moved, and more expeditiously and less expensively installed and removed.” The reader will be able to form his own conclusions from the evidence which has been laid before him. From a German official of public works, Herr Fülscher, cornes a novel suggestion for compound gates. Since wood is durable under water and perishable above, while for iron and steel the conditions are reversed, Herr Fülscher advocates the employment of each material in the situation which is particularly favourable to it, so that the lower part of a gate would be of wood and the upper part of iron. The idea is ingenious and plausible, but no attempt has yet been made to carry it into effect, and there are several Serions difficulties in the way of its realisation. It would manifestly be an * Min. Proc. Pist. C.E., vol. xcvii., p. 386.