MINING METHODS. 323 



is allowed to flow. This will carry stones up to 7 or 8 inches in 

 diameter into the tail race at the foot of the sluice boxes. 



Steps.— The steps in the boxes are movable pieces of timber of 

 say, 4 XlJ inches section. They are put in at intervals of 20 feet 

 and added to as concentrate accumulates. 



Cleaning-up.— With ordinary material the box should take about 

 a month to fill, when it must be cleaned up. It is unprofitable to 

 try to make a concentrate richer than 5 to 10 per cent., as to do 

 so increases the risk of loss. Channelling or guttering is the mam 

 source of loss. It is caused by inattention and also by allowing 

 the full body of clear water to flow through the box when the break- 

 ing of ground by the monitor is suspended. Six men per shift 

 will keep the boxes in good order. 



If much tough clay has been sluiced the material may set hard 

 m the boxes, and, as the one described may hold 700 tons of stuff, 

 the clean-up is not an easy matter. At Kanbauk a modification 

 of the Briseis (Tasmania) practice is followed. A 6-inch elevator 

 is erected at the foot of the boxes so that its swivelled suction may 

 dip into the bottom of any compartment. The contents of the 

 box are sluiced to this with a 1J inch nozzle on a fire hose. The 

 concentrates, now mixed with a large volume of clean water, are 

 elevated into a clean-up box with a width of 4 feet, set parallel 

 to the main box but running in the opposite direction, so that the 

 tailings deliver back into one of the compartments of the mam box 

 and the chances of loss are reduced. 



The concentrates, enriched to about 40 per cent, by this treat- 

 ment, are shovelled out on to flat sheets and carried into the dressing 

 shed ' where they are further treated by hand. 



' f a ;i , n<T6 _For a flow of 20 cusecs, a tail race 4 feet wide is 

 sufficient It should have a grade of at least 1 in 25 and should also 

 be paved with wooden blocks or large hard stones. At Kanbauk 

 the tail races are paved with 9 inch blocks cut from round jungle 

 timber and laid on end. These have a life of three seasons. 



Hydraulic elevators.- Hydraulic elevation becomes necessary when 

 tailings can no longer be disposed of by gravitation. The hydraulic 

 elevator is a cheap and easily erected machine but it is very inefficient, 

 Continuing the case of 20 cusecs of water under an affective 

 head of 350 feet, and supposing that it is necessary to elevate 6o 

 feet for this lift the average efficiency of a new elevator may be 

 taken at 15 per cent, but as the throat of the machine wears away 



