2g6 SEWAGE AND ITS PURIFICATION 



gear, carriage, and erection, the cost is as follows : 25 ft. diam.,. 

 £^S j 50 ft. J £S5 J 100 ft., pTioo ; and so on in proportion, 

 "' The least head required is that which would cause the sewage 

 to gravitate to a point 6" above the centre of the surface of the 

 filter bed, but it is advisable to allow not less than 9" for a 

 constant uniform supply." If the supply is variable, the head 

 must be greater in proportion. The maximum head is not 

 stated, but apparently there is a limit beyond which there may 

 be an overflow of sewage from the central air-lock. The 

 apparatus is intended to work with an automatic flushing tank 

 when the rate of distribution falls below a certain quantity per 



Fig. 44, — Patent Revolving Spreader, in conjunction with a Patent 

 Automatic Measuring Valve, for either Continuous or Inter- 

 mittent Feeding (Mather and Platt). 



sq. yd., and under these conditions it is said to distribute as 

 much as 100 gals, per sq. yd. per 24 hrs. The effect of wind 

 pressure is naturally much the same as on other types of the 

 Barker's Mill, and if this be taken into account at least 12" of 

 head would be required to drive it against an ordinary wind. 

 Messrs. Adams are now constructing distributors to be driven^ 

 by power, which appears to indicate that for the larger sizes, at 

 all events, automatic distributors cannot be entirely relied on. 

 In one of the latest forms the arms are fitted with two rows of 

 spray holes, the lower for the minimum, and the upper for the 

 maximum discharge. In this case the minimum and maximurm 



