LOCKS. 167 



is very difficult to keep the plank in line and to surface during ramming, the appear- 

 ance of the wall suffering thereby, and because the concrete can only be built in small 

 layers, instead of in large blocks or as a monolith. 



With outside posts this difficulty is greatly reduced, and better results obtained 

 in other ways. Much lighter timbers are used now for this class of form than in the 

 first locks, where the posts were 8 inches by 10 inches, 4 feet apart, and the lagging 4 

 inches thick. For the same purpose posts are now used 3 inches by 8 inches, 5 feet 

 apart, with lagging 2 inches thick, and the results are equally satisfactory. Where the 

 lagging is of if -inch by 12 -inch plank, the posts should not be more than 4^ feet apart, 

 and if it is of if -inch by 8-inch plank, they should not be more than 4 feet apart. 



The principal object to be secured, where it is desired to obtain a good appearance, is 

 thorough stiffness in the posts and braces. The old idea that the concrete brought against 

 these a liquid pressure has been proved by experience to be erroneous, and the real force 

 which distorts the timbers comes from the rammers, and to withstand it a great deal 

 of stiffness is required. 



All lagging plank, where a good surface is desired, should be seasoned and dressed 

 on all sides, and the edges of the posts should also be dressed. The additional cost 

 is small, and unless it is done bad joints and gaps will be visible everywhere. For 

 the same reason old lagging, unless well cleaned and retrimmed where needed, should 

 not be used for the best class of finish. The plank may be from 6 to 12 inches wide, 

 some engineers preferring a narrow width on the ground that many close joints 

 are less noticeable than a few wide apart, and also that narrow plank do not warp 

 as much as wide ones. Yellow pine appears to be more satisfactory for this use than 

 other kinds of timber, as it is less affected by the moisture, and less liable to warp and 

 shrink. Matched lumber has been tried once or twice with a view to lessening the marks 

 of the joints, but with indifferent results. 



The raggedness of the vertical and horizontal joints of the blocks, where the wall 

 is built in sections, may be avoided by the use of triangular strips of wood, of cross- 

 section like a 45-degree triangle, and about f of an inch on the sides. One of 

 these is placed at each corner of the block and the facing rammed around it as the 

 block is built up, and others of the strips are laid along the top edges when they are 

 finished for the time being. When the adjoining block is built up, or when the first 

 block is continued, similar strips are placed against those in the facing, and the result 

 shows, when the forms are removed, as V-shaped grooves of smooth outline. Care 

 is of course needed in making and setting the strips, so as to have them all of one size 

 and placed true, and the horizontal joints must all occur at similar levels. 



In one or two cases the lagging has been lined with galvanized or plain sheet iron, 

 well greased, and this is said to have given an -excellent finish, doing away with the 

 joint- and grain-prints which are inseparable from wooden lagging, and which cause, on 

 the score of appearance, one of the principal objections urged against concrete locks. 



It is probable that a combination of galvanized sheet iron and triangular strips 



