igo HARBOUR CONSTRUCTION. 



considerable depth of water is injurious, and should be dis- 

 couraged as much as possible. Several years ago I inspected a 

 mound of bag-work which was being deposited from hopper 

 barges in this manner, and I noticed that the bags were much 

 broken, the mound presenting somewhat the appearance of a 

 shingle beach, in consequence of the sea having washed out the 

 cement and dispersed much of the shingle of which the concrete 

 was composed. 



When a box containing a bag-block has been lowered to 

 within a short distance of the bottom, its position should be 

 adjusted by divers unless some other means can be devised 

 so that the bag may, on the catch being released, fall into the 

 place assigned for it. 



The canvas, which in large bags is sometimes used double, or 

 strengthened by means of belts of the same material, effectually 

 prevents the cement from being washed out by the action of the 

 sea, unless broken by rough usage. 



Bag- work may be roughly levelled at the time of deposit, by 

 the divers using " beaters," or preferably by weight applied from 

 above, in obedience to the divers' signals. Squeezing by applica- 

 tion of weight is better than beating, but as little of either 

 should be resorted to as possible. 



On no account should concrete be meddled with when once 

 it has commenced to set, until it has become sufficiently hard to 

 admit of being dressed off by picks, without injury to that which 

 remains. This may be done before the concrete is so hard as to 

 entail much labour. 



Bags should be made slightly larger than the boxes, so as 

 to give the concrete, when deposited, freedom to spread and 

 conform to irregularities of the bottom, and also to allow for 

 the shrinkage 1 of the canvas when wetted. The amount of 

 slackness to be given will vary according to the proportions 

 of the bags, shallow bags requiring less in proportion than 

 deeper ones, on account of the concrete having less tendency 

 to spread. The correct amount to allow is best ascertained by 

 experiment. 



The proportions of materials most suitable for making 



1 This varies in different qualities of canvas. It is very small in the direction 

 of the " weft," or from selvedge to selvedge, but in ordinary jute sacking it amounts, 

 on an average, to somewhat under half an inch in a foot along the " warp," or in 

 the direction of the length of the piece. 



