u6 HARBOUR CONSTRUCTION. 



moistened to some extent. The quantity of water should, how- 

 ever, be carefully regulated, because if used in excess the strength 

 of the cement will be impaired, whereas if too little be used the 

 concrete will be porous and liable to injury by the action of the 

 sea-water. 



Mixing Concrete by hand. In mixing concrete by manual 

 labour, the usual course pursued is as follows : 



Upon a smooth timber platform, say 14 to 16 feet square, 

 which is termed a "banker," the dry materials are measured 

 in gauges. These may be described as bottomless boxes, which, 

 when lifted, leave upon the banker the material that has been 

 gauged. This gauging should be done at the end of the banker 

 opposite to that from which the mixed concrete will be delivered. 

 The mixers should then arrange themselves in two rows facing 

 each other. The cement having been evenly distributed over 

 the gravel, or broken stone and sand, as the case may be, the 

 first pair of men pass the material on to the second pair, at the 

 same time giving their shovels a twist so as the better to mix it. 

 It is thus passed on from pair to pair the water being added, 

 through a rose, between the second and third pair until it 

 reaches the other end of the banker, by which time all the 

 ingredients should be properly incorporated, and the concrete 

 ready to put into the work. 



The materials are usually turned twice in the dry state, and 

 twice after the water has been added, which represents four 

 pairs of mixers ; but unless the men are properly trained, and 

 give the necessary twist with their shovels in passing the material 

 on, double that amount of shovelling or handling of the material 

 will not produce good concrete. 



Concrete-mixing by Machines. Various machines have been 

 designed for mixing concrete mechanically. They may be 

 generally divided into two classes, namely, those in which the 

 process is intermittent, and those in which it is continuous. 



Of the former, Coode's and Messent's may be cited as 

 examples. 



Coode's mixer is composed of an inclined cylinder with an 

 axle, upon which it revolves, passing through it diagonally, and 

 resting upon side frames. There is a door at one or both ends 

 of the cylinder, through which it is charged and the concrete 

 delivered when mixed. The eccentric arrangement of this 

 cylinder upon its axle is such that, when made to rotate, its 



