WATER FILTRATION. XLIX. 



scraping, of having filtered water introduced from below 

 until the sand is filled to the surface. 



10. The velocity of filtration in each single filter shall be 

 capable of being arranged to give the most favourable 

 results, and shall be as regular as possible, quite free from 

 sudden changes or interruptions. Ou this account reser- 

 voirs must be provided large enough to balance the hourly 

 fluctuation in the consumption of water. 



11. The filters shall be so arranged that their working 

 shall not be influenced by the fluctuating level of the water 

 in the filtered reservoir or pump-well. 



12. The loss of head shall not be allowed to become so 

 great as to cause a breaking through of the upper layer on 

 the surface of the filter. The limit to which the loss of 

 head can be allowed to go without damage, is to be deter- 

 mined for each works by bacterial examination. 



13. Filters shall be constructed throughout in such a way 

 as to insure the equal action of every part of their area. 



14. The sides and bottoms of filters must be made water- 

 tight, and special pains must be taken to avoid the danger 

 of passages or loose places, through which the unfiltered 

 water on the filter might find its way to the filtered water 

 channels. To this end special pains should be taken to 

 make and keep the ventilators for the filtered water chan- 

 nels absolutely tight. 



15. The thickness of the sand-layer shall be so great that 

 under no circumstances shall it be reduced by scraping to 

 less than than 30 centimetres (12 inches), and it is desir- 

 able, so far as local conditions allow, to increase this mini- 

 mum limit. Special attention must be given to the upper 

 layer of sand, which must be arranged and continually kept 

 in the condition most favourable for filtration. For this 

 reason it is desirable that, after a filter has been reduced 



4— July 21, 1904. 



