i4 THE IMPROVEMEXT OF RIVERS. 



instance the under side of the bottom beam, which could only IK- cleaned and repainted 

 with great difficulty, if at all.* For tin-so reasons the parts constantly under water 

 should possess a large excess of strength, and all the metal in them should be at least 

 | inch in thickness, 'the lower part of a wooden gate will usually outlast two upper 

 parts, but of steel gates it is probable that the reverse will IK- found to be true. 



On the continent of Europe steel has replaced wood almost entirely, even in very 

 small gates, but in England wood is still largely employed. 



Vertical and Horizontal Framing. Metal gates for locks of ordinary size may be 

 divided into two classes, those with vertical framing, and those with horizontal fram- 

 ing. In the former class the beams supporting the water stand vertically, and are 

 supported by the miter-sill at the bottom and by a horizontal girder at the top. In 

 the latter class the beams are horizontal, as in a wooden gate, and are framed into 

 vertical heel- and toe-posts, which in turn transmit the strains to the walls and to the 

 opposite leaf. 



There is also a mixed type in which vertical girders are employed to assist in carry- 

 ing the loads from the plating to the horizontals, the latter forming the main supports. 

 With this construction both horizontals and verticals act in resisting the pressure, 

 and the precise loading of each can only be approximated by a tedious mathematical 

 investigation! For this reason many designers assume that all the pressure is resisted 

 by the horizontals, and make each vertical strong enough merely to carry the pressure 

 to them from its own panel. This type is also more complicated in construction 

 than a plain system of horizontal or of vertical framing, and as a rule is therefore not 

 economical. 



Where a gate is very shallow in proportion to its length, economy of metal favors 

 the use of vertically framed gates. Some engineers, however, claim that the saving is 

 comparatively small, even in extreme cases, and does not offset the disadvantages. 

 These are, that the top horizontal girder brings a heavy strain high up in the wall, and 

 that the recess in the coping necessary for it when the gate is open, reduces the width 

 of surface required by the lock-tenders for the proper operation of lock, or else neces- 

 sitates a wider wall in order to avoid such a reduction. To these objections, it might be 

 added that when the lower part of a vertically framed gate is rusted away the entire 

 leaf must be renewed. With horizontal framing the gate could be cut in two at the 

 beam nearest the water, and a new bottom riveted on, the upper portion being still 

 retained in service. 



Vertically framed gates are only applicable to new locks, on account of the girder 

 recess above mentioned. 



Design. The economical design of a steel gate requires few members with wide 

 spans between, thus concentrating the strength and allowing a good thickness of 



* One of the most effective coatings for jirescrviii}; ironwork under water is the ordinary red- 

 lead and oil paint, while pitch has also given excellent results in some localities. In certain of 

 the French seaport locks the metal is galvanized, which process is said ',o add about ten per cent 

 to the cost. 



