PURIFICATION OF WATER FOR DRINKING PURPOSES 161 



value of which it was desired to compare. The tanks 

 within the building, oil the other hand, were of galvan- 

 ized iron, of about the same depth as the larger wooden 

 tanks, but with only a diameter of twenty inches, giving 

 an area of surface one-hundredth of that of the large 

 tanks, or one twenty-thousandth of an acre. They 

 were all provided with a substratum similar to that 

 described for the large tanks, and were filled with 

 various depths and varieties of materials. The sewage 

 supplied to the tanks was abstracted from the main 

 sewer of the city of Silf. Lawrence, about one thousand 

 feet above its outlet, and above the entrance of streams 

 from the manufacturing establishments, and may be 

 regarded, therefore, as typical city sewage obtained 

 from the shops and dwellings of perhaps ten thousand 

 persons. It must be borne in mind, however, that 

 American sewage is much weaker than that with which 

 we are accustomed to deal, it being largely diluted with 

 pure water. An American sewage stronger even than 

 usual would contain as much as 998 parts of pure 

 water per 1,000. 



It would be impossible here to enter into the details 

 of the experiments made upon each individual tank, 

 but we have selected one of the large tanks as an illus- 

 tration of the scale upon which the investigations 

 were made, whilst the general results obtained with the 

 different tanks will be shortly summarized. 



Tank No I. then, after being packed with the sub- 

 stratum of material previously described, was filled five 

 feet in depth with very coarse, clean mortar-sand, the 

 total amount, including what was used in the substra- 

 tum, being about 9,000 gallons. Sewage was first 

 applied intermittently on January 10, 1888. Previous 

 to that time water had been passing through the filter 

 at the rate of 1,000 gallons a day for a month. The 



M 



