FOUNDATIONS. 



267 



Li lie for lowering bag, 



means of an ordinary spirit-level. This having been done, coach- 

 screws are inserted through the eyed ends of the bolts, for the 

 purpose of holding the planks in place. 



" Before lowering the planks for fixing, the surfaces which will be 

 in contact with the concrete are covered with jute sacking, a flap of 

 the same material, from 15 to 18 inches wide, being left so as to 

 extend under the concrete and prevent its escape. A length of about 

 20 feet of foundation is usually dealt with at one time ; but this, of 

 course, is merely a matter of convenience, which may be regulated as 

 circumstances require. 



" Where the mass of concrete required to level up a foundation is 

 large, it may be composed of, say, 4 or 4 parts of good clean sand 

 and gravel to 1 part of Portland cement, the top being finished off 

 with a layer, about 6 inches in thickness, of rich concrete composed 

 of 2 parts of fine gravel and 

 sand to 1 part of Portland 

 cement. W here the thickness 

 of the concrete is small, only 

 the richer compound should 

 be used. 



" The concrete is lowered 

 to the divers in bags specially 

 designed by Mr. Shield for the 

 purpose. These are illus- 

 trated by Fig. 87. The mouth 

 of the bag is closed by one 

 turn of a line which is pro- 

 vided with loops through 

 which a hard wood tapered 

 pin passes. This is attached 

 to the tripping line, as shown 

 in the engraving. The folds 

 of the bag hold the pin in 

 position, but a slight pull is 

 sufficient to withdraw it and 

 release the contents of the 

 bag. The bags are made slightly tapered, say 3 inches on a side, so 

 as to facilitate the discharge of the concrete. They should be lowered, 

 mouth downwards, until they almost touch the bottom. The divers 

 then place them over the site where the concrete is required, and on 

 giving the signal ' All right/ the tripping pin is withdrawn and the 

 contents of the bag deposited. 



" The loss of cement by this system is so small that the water is 

 scarcely discoloured. The bags usually contain 2 cubic feet each, 



FIG. 87. Tripping bag. 



