DOCK GATES. 221 



simplicity of construction, and freedom from the attacks of the 

 worm. Another and important advantage claimed for metal 

 gates is the buoyancy capable of being given to them by the 

 introduction of air-chambers of sufficient capacity to float the 

 gates. The working weight on the bottom, and consequently 

 the strain upon the hinges, being regulated by ballast, 1 or the 

 air-chambers may be of such a capacity only that there may be 

 a preponderance of weight to prevent the gate lifting. 



At the Bute Docks, Cardiff, a composite gate has been 

 adopted with success. This method of construction consists of 

 a wooden skin, supported on plate girders, strengthened by 

 vertical and diagonal ribs. It is claimed that a leak can be at 

 once detected and easily repaired, either by fresh caulking or by 

 cutting out and replacing a plank, whilst the dimensions and 

 position of the girders can be exactly adjusted to the requirements 

 of both theory and practice. 2 



Form of Gates. The simplest form of construction is a 

 straight gate, with continuous horizontal beams meeting with 

 a mitred joint on the centre line of the entrance, and, for small 

 spans, is perhaps the best that can be adopted. 



For wide openings, however, the conditions are different, and 

 elaborate investigations point to the conclusion that the nearer 

 a gate approaches a true circular arc the stronger it will be ; or, 

 expressing the same thing in another way, " with a circular form, 

 equal strength can be obtained with less expenditure of material." 



Mr. Barlow, 3 Sir F. Bramvvell, 4 Mr. Kingsbury, 5 and others 

 refer to several important advantages attaching to curved gates 

 as compared with straight ones ; amongst others, that when 

 closed, and forming a true segment of a circle, they are not 

 subject to transverse strains, and therefore not to any liability 

 to distortion which such strains alone can produce, but that the 

 whole of the material in the gates is in compression, and that 

 they require less material than straight ones. 



A form approaching that of a Gothic arch is advocated and 

 adopted by some engineers, on the ground that less material 

 is required than for a true cylindrical form, also that the latter 

 form is liable to flexure, by which the character of the strains 

 are altered, a defect which does not obtain in a Gothic or pointed 

 form of gates. 



1 M.P.I.C.E., vol. xxxi. p. 357. 2 Hid., p. 338. Ibid., vol. i. p. 74. 



4 Ibid., vol. xxxi. p. 343. * Ibid., vol. xviii. p. 445. 



