WATER FILTRATION. XXXIII. 



admitting of the air finding vent at the crown of the ridges 

 as the water level rose. The filter bed, not being subdivided, 

 does not admit of one section being thrown out of work for 

 cleaning while the other keeps up supply. Since my visit, 

 the Water Board has divided the area into three sections 

 by dividing walls, thus admitting of filters being worked as 

 in Great Britain and Europe. There was nothing novel in 

 either the removal or the washing of the sand. The cost 

 of maintenance of the filter was given in 1900 as 7'02 dollars 

 per million U.S. gallons; in 1904 the cost was 7*07 dollars 

 per million U.S. gallons. 



The author had an opportunity of contrasting the raw 

 water of the Merrimac with the resultant filtered water, 

 and when it is taken into consideration that there was no 

 subsiding or sedimentation reservoirs, or roughing filters, 

 the quality of the filtered water was remarkable. The 

 percentage of bacteria removed was as high as 99°/°. This 

 work has been referred to on account of the novelty of 

 construction — the engineer having struck out on bold lines 

 with satisfactory results — also to shew how a community 

 can cope with an alarming epidemic of typhoid through 

 drinking the polluted waters of the canal and river by 

 factory hands and others. It is stated that the actual 

 benefit derived from the use of the filtered water as against 

 the former use of that from the river, may be fairly repre- 

 sented by the prevention of deaths of 40 persons out of every 

 100 dying from typhoid fever in Lawrence during the six 

 years prior to the construction of the filter. 



Rapid or Mechanical Filtration. 

 This method has been well denominated "the American 

 system." Mechanical filtration has gradually developed 

 from the filtration of polluted water from paper factories 

 for re-use, to its present position of filtering river water 

 of doubtful quality for human consumption. One company 



3-July 21, 1904. 



