VARIOUS TYPES OF BREAKWATERS. 185 



the openings not be too large, the canvas lining referred to 

 below will be sufficient to prevent the escape of the concrete. 

 Should the rock surface slope quickly, it may be necessary to 

 bench it by blasting or otherwise. 



The sides of the casing should be supported by means of 

 through cross-ties (usually f -inch round iron), in order to prevent 

 bulging by the lateral thrust of the liquid concrete; and, in 

 order to prevent loss of cement by the wash of the sea, the 

 sides and a portion of the bottom of the casing should be lined 

 with strong jute canvas, secured to the planks by clout nails. 

 Should the bottom be porous, as would be the case in founding 

 upon a rubble base, its entire surface should be covered with 

 canvas ; indeed, in such a case a large bag, made to loosely fit 

 the casing, should be used. 



The frames are frequently anchored to the bottom by means 

 of stay chains attached to lewises, union^ screws being provided 

 for tightening them up. 



After the concrete has been deposited, its surface, if within 

 reach of the waves, should be protected from their fretting action 

 by one or other of the methods described on pp. 258 and 259. 



In founding upon a level base, the frames may be used over 

 and over again; but when the ground is uneven it is often 

 necessary to cut timber for each frame, the cost of the work 

 being thus increased, not only by the unavoidable waste of 

 timber, but also by the extra labour entailed. 1 



Furthermore, it is necessary to use a larger proportion of 

 cement in concrete which is deposited under water in this 

 manner than in blocks which are made and allowed to harden 

 before being put into the sea, the cost of the work being thus 

 still further increased. 



The difficulty of usefully employing men engaged on this 

 class of work when it is interrupted, either by rough weather 

 or in some other way, is not the least of its drawbacks, and 

 in this respect it contrasts unfavourably with the block system 

 of construction. 



From what has been said, it will be seen that work performed 

 in this way in an exposed situation may prove very costly, 

 especially when it happens to be below low water, and divers 

 have to be employed. 



1 When the foundation is fairly level, horizonal boarding is sometimes used 

 in preference to vertical, 



