DOCK GATES. 231 



fixed a strong toothed rack working in a pinion at the end 

 of a train of gearing, the first motion of which was a screw 

 and worm-wheel. By means of this gearing, the power of the 

 men was communicated to the gate through the beam or stay. 

 The action was so perfect that the gate would remain stationary 

 in any intermediate position of the gearing when let go, even 

 when there was a considerable amount of disturbance in the 

 harbour. The span of the gates was 50 feet, and the shock of 

 the waves was occasionally very great. 



By this simple contrivance the gates were opened and closed 

 without difficulty when there was a considerable amount of 

 undulation in the harbour. 



Stay Gates. These consist of small gates which, when the 

 large gates are closed, take a position as shown in the diagram 

 at a (Fig. 214). Fitting tight against the back of the gates, 

 they form shores or struts, and so keep 

 them steady when a swell sets in against 

 them. These stay gates are usually 

 worked by chains in a manner similar 

 to the main gates. 



Dunkirk Dock Gates. M. Guillain, the 

 engineer of the Port of Dunkirk, has Fm 211 



expressed an opinion 1 that a mode of con- 

 struction which throws upon the face of the sill the greater 

 part of the total strain due to water pressure, will produce 

 considerable economy of material, when the length of the leaf 

 is greater than the height. It is then necessary to have a few 

 horizontal flanges and vertical members properly proportioned ; 

 an extreme case being that in which the gate would comprise 

 only a top flange, vertical girders, and a bottom flange resting 

 against the sill, and sheeting stiffened by ribs. In this case, 

 the top flange would bear one-third of the maximum total 

 pressure, while the sill would bear the remaining two-thirds by 

 means of the bottom flanges. 



This system was adopted by M. Guillain in the iron gates for 

 the new Dunkirk Docks (Figs. 215 to 218). The width of the lock 

 entrance is 60 feet, and the rise of the sill one-sixth ; the gates are 

 27 feet high above the sill ; the outer surfaces are flat, so that 

 each leaf has a total length of 38 feet 4 inches, by a height of 

 27 feet 9 inches, and a thickness in the middle of 3 feet 7 inches. 

 * M.P.I.C.E.. vol. lix. p. 29. 



