CATTLE DIPPING TANKS 237 



A site in very clayey ground should be avoided, as nearly pure clay 

 contracts too much in dry weather and expands again in wet weather. 



The excavation should be very carefully performed, as, if too much 

 earth be removed, much more concrete than the amount estimated will be 

 required. The side-slopes may be formed to a wooden templet of such a 

 shape that when one edge is set vertically, by means of a plumb-rule, the 

 other edge gives the slope of the sides. It is better to excavate too small 

 at first and then trim off carefully afterwards, than to run the risk of 

 removing too much soil. 



Hard-core Cushion. — Sufficient ground should be excavated to allow 

 of a 6-inch layer of hard core (consisting of stones about the size of a 

 third of a brick) being laid down below the floors of the swim and out- 

 slope. Frequently stones can be picked up on the veld near the 

 site, which, when broken across with a hammer, are about the correct 

 size. Breaking each stone across exposes a fresh surface on which the 

 concrete of the floor gets a good grip. The hard core should be well 

 rammed. The race is founded on a similar layer of hard core. 



Materials. — The concrete may be mixed in the proportions 1 part 

 cement, to 2 parts clean sand, to 4 parts broken stone. The sand 

 should be clean and large in the grain, and, to make sure of good results, 

 river-sand ought to be used. The broken stone should be small, so as 

 to pack closely round the reinforcement iron. It should, therefore, be 

 capable of passing through a screen of |-inch mesh. 



The concrete should be mixed rather wet. If it runs off the shovel 

 unless handled quickly, it will be of about the correct consistency. It 

 may be mixed on a platform of wood or iron sheets. The correct pro- 

 portions of sand and cement should be measured out by the use of a 

 cask or measuring box (without top or bottom) on the platform, and 

 should be turned over three times dry, or until the mixture assumes 

 a uniform colour throughout. 



The measured quantity of broken stone is then added. 



The mixture is now wetted by water from pails, and is turned over 

 three times wet. Care should be taken that some of the fine cement 

 is not washed away while water is being added. Thus the mixing 

 platform should be practically water-tight. 



In turning the mixture, either dry or wet, it should be shovelled 

 from one pile into a distinct and separate new pile, and then shovelled 

 back again on to the site of the original pile. By proceeding in this 



