LVI. C. W. DARLEY. 



to the inner and outer faces of the wall below. The attachment 

 is by J in. bolts screwed at both ends, a bolt and nut is built 6 in. 

 into the concrete at intervals of 10 feet horizontal and 3 ft. vertical. 

 When a course is completed, the mould-boards and profiles are 

 lifted 3 ft.; unscrewing the bolts leaves the nuts in the wall, and 

 the bolt-holes are then filled with mortar. The mould-boards are 

 curved to the mean radius of the face of the wall. On the down- 

 stream or concave face the radius increases as the work is carried 

 up, and the mould-boards gradually separate. Filling pieces have 

 to be inserted between the mould-boards; on the upstream or 

 vertical face this does not occur. 



The proportions of the ingredients of the concrete used in the 

 body of the wall are — 1 cask of cement, 4 J cubic feet; 12 cubic 

 feet of sand; 10 cubic feet of shivers, \ inch gauge; 13 cubic feet 

 of metal, 1 \ inch gauge. A six inch facing is used on the water 

 side of the wall composed of 1 cask of cement, 4^ cubic feet; 10 

 cubic feet of sand; 10 cubic feet of shivers, J inch gauge. 



Plumstones to the maximum size that can be handled by two 

 men are built into the body concrete as closely as will allow of 

 proper ramming and packing round them. They can be most 

 efficiently bedded on their thinnest edges, and should have their 

 greatest length radial to the curve of the wall. With careful 

 packing, and selecting stones of square dimensions, it is possible 

 to get 45% of stone into the work, but in dams of small radius 

 and therefore thin walls 33% of plumstones should only be 

 calculated upon. These small stones have been found to be the 

 most economical size to use; as the dams built for the supply of 

 water to country towns are rarely large enough to bear the expense 

 of providing plant for lifting larger weights the economy of the 

 use of plumstones is obvious. 



The practice has been to use the richer concrete on the water- 

 face to make a water-tight wall. Experience has shewn the 

 impermeability of the structure depends almost wholly on the skin 

 worked up on both faces of the concrete. This skin consists of a 

 thin layer of neat cement, which is obtained by working a spade or 



