i8 THE IMPROVEMENT OF RIVERS. 



excellent meth<xl of construction, as it saves material ami woight, and also reduces. 

 the buoyancy of the gate in high water. The beams are usually made of the same 

 width and depth as those below, and placed at distances apart which will make each 

 one support the same unit stress. 



Sizes of Beams. The following are the horizontal widths of beams used in actual 

 practice in certain horizontally framed wooden gates. The timLcrs were of Georgia yel- 

 low pine. 



For lock chambers 27 feet wide, and heads up to 10 feet, the beams are 12 inches 

 wide. 



For lock chambers 36 feet wide, and heads up to 16 feet, the beams are 15 inches wide. 



For lock chambers 52 feet wide, and heads up to 15 feet, the beams are 18 inches wide. 



In some cases two beams, placed side by side and keyed together, have been used 

 instead of one single large beam; thus we find examples where two beams 9 inches 

 wide, and other examples where two beams 12 inches wide, have been used instead of 

 one single beam 18 inches wide. This practice, however, is not a satisfactory one, as 

 it takes more labor in construction and depends for proper strength on the exact fitting 

 and duration of the keys. The cost of two small timbers is of course less than that of 

 one large one, but the additional labor required in framing and handling the former 

 will more than offset this. Experience with double- and single-timbered gates on the 

 same river has shown, moreover, that the former are more easily injured by shocks 

 and accidents. 



Kind of Timber. In most gates white oak has been the timber used, and where 

 sound sticks can be obtained, and especially if they have grown on bottom lands, 

 there is no better material. Yellow pine has been largely used of late years, as oak 

 is becoming scarce ; it has proved very suitable, and costs a little less in the framing 

 than oak. White and Norway pine are also good, but they are not often used except 

 in Northern streams on account of the expense. Hemlock is not suitable, as it rots 

 easily. 



On the Manchester Ship Canal, in England, Demerara greenheart was used in 

 preference to metal in all the gates. This is an unusually durable timber, although 

 it may be questioned whether this fact would compensate for the advantages to be 

 gained in large gates by the use of steel. 



Assembling. Where gates are built of horizontal beams of the full length these 

 are held together by 1} or i^-inch bolts, running vertically through their centers, 

 from the top beam to the bottom one, or by metal straps placed on the outside, and let 

 in flush with the faces of the gate. Where the latter are used those at the heel and 

 toe should be provided with turnbuckles for use in cramping the timbers together dur- 

 ing construction. In each case diagonal straps must be used to hold up the gate; 

 these are preferably formed of eyebars provided with turnbuckles, one end passing 

 over a pin in the bonnet, the other over a similar pin near the lower end of the toe. 



If the timber is to be dressed at the site, the pieces should be ordered \ inch 

 larger than the finished size, to allow for planing. With oak or other timber liable to 



