CAISSONS. 247 



of two or three inches from the stop. Vertical wires with 

 heavy weights attached are then dropped at the sides, reaching 

 to the bottom of the entrance, or as low down as possible, with 

 the weights immersed in buckets of oil to steady them. A 

 second horizontal wire is then fixed at the bottom, with one or 

 more intermediate wires according to the shape of the entrance. 

 Wires are then fixed at the top and bottom to suit the batter of 

 the side walls, and, if necessary, others to suit the curve of the 

 haunches. From these wires the work can be spotted in to 

 gauge at very short distances, and the face finished off with 

 straight-edges, which should be as long as possible. In working 

 round the curves of the haunches where only short straight- 

 edges can be used, the tendency is, unless great care be taken, 

 to work a little hollow, with the result that leakage takes place 

 between the stone face and the timber keel-pieces ; to avoid this, 

 the stone faces round the short curves should be worked a 

 shade full. 



The advantages attaching to caissons are, the saving of room 

 in shortening the length of the entrance, particularly where 

 double gates would be required ; the facility with which they 

 can be maintained and repaired as compared with gates ; the 

 power they give of examining the sill and masonry of the 

 entrance by their removal temporarily to an outer stop provided 

 for the purpose; and the extra length which such removal 

 gives on the floor for docking purposes whenever it may be 

 necessary to deal with an extra long ship. Hollow quoins which 

 are costly and difficult to work, and are seldom water-tight, are 

 dispensed with. They afford a ready means of carrying an 

 ordinary road or railway across a dock entrance, thus avoiding 

 the heavy additional cost and inconvenience of swing- bridges, 

 while the dock-side is kept clear of all obstructions, a most 

 important point to be kept in view. When the traffic is heavy, 

 one of the conditions is that the deck of the caisson must be 

 level with the coping on either side of the entrance. 



In the case of the floating type, this is effected by adjusting 

 the total height of the caisson, when sunk to its bearings on the 

 invert, to coincide with the coping level 



In the sliding or rolling type, the object may be effected by 

 making the roadway deck capable of being lowered sufficiently 

 to admit of the caisson passing under the deck of the chamber, 

 by lowering the caisson bodily, by sinking it to a sufficient 



