344 M0D2RN' METHDDS OF WATER PURIFICATION. 



we have to choose in the near future on which hnes our ex- 

 tension of fihers is to be made, if necessitated by the fikers 

 being taxed beyond their capacity by increased demand for 

 water, after the necessary experimental work is done that 

 needs doing- to make quite certain, a far wider experience than 

 previously will be available, experience which is being added 

 to every year in different lands. 



One point to remember is this, that if the coagulation is done 

 so as to be fitting the needs of the water, it will not be a con- 

 stant equal amount as at present that will be correct, but an. 

 amount varying daily according to requirements, but subject 

 to a definite equivalent, which might be advantageously esti- 

 mated by a Hazen's turbidity measuring tube, by the Engineer 

 at the waterworks, and the amount of precipitant added would 

 vary with the reading and be subject to the City Engineer's 

 orders. If the waterwas so watched and treated, filters would 

 by that means be expanded in efticiency equal to twice or thrice 

 their area, and still give the same quality of water, in this way, 

 that whereas the filters are set to give Koch's standard of 4 

 vertical inches, or 2 gallons per hour per square foot, they 

 could, by carefully adapted precipitation, give a yield of 4 to 

 6 gallons per square foot per hour. If this is possible the 

 filtering capacity is doubled or trebled. 



I would recommend to local authorities that they obtain such 

 appliance, and have experiments made on the filter bed to find 

 out if the bacterial efficiency be equal to the present if the head 

 is so increased as to give a double or treble yield. This is a 

 direction in which experiment will give interesting results, and 

 if they are favourable and can be kept continuous you do not 

 need to increase your filters unless a large expansion is antici- 

 pated. But, I again repeat, quality and efficiency and purity 

 are the first desiderata and the chief aims. 



I have quoted Professor Smith, of King's College. London, 

 in the intention to present, not only one side, but both sides, 

 when he speaks of sand filters as the only safe method. Else- 

 where he writes, 



" It must be very strongly insisted upon that much more experience is 

 desirable before it could be recommended to replace sand filters by 

 any of these mechanical filters." 



Not only he but others also hold those views, and are entitled 

 to respect and to demonstration when their views are called 

 into question, in view of the great experience which they have 

 acquired. But there are those who have not that experience, 

 and who, for want of it, have their vision narrowed and 

 blinkered. 



I have seen results with filters which surpassed anything 

 that I had been prepared to find, e.g., the Candy at Hastings 

 with iron-red water, and the American or Jewell system at 

 numerous installations I have visited. 



The results attained in Alexandria are first-class, and will 

 bear favourable comparison with the sand filter figures as 

 quoted above from W. R. Smith's paper. The water for 

 Alexandria is drawn from a canal which i's fed from the Nile,. 



