162 MICRO-ORGANISMS IN WATER 



filtration was much interfered with by snow and ice ; on 

 .one occasion a depth of snow and ice of one foot was 

 found above the sand, and ten inches of frost within 

 the sand ; but although the latter was so frozen that 

 no opening could be found for a fine steel point, yet it 

 nevertheless allowed liquid to pass through it. 



The table on following page shows the number of 

 micro-organisms found in a c.c. of the sewage applied, 

 and in the effluent from January 3, 1888, when the 

 tank was first set to work, to October 25, 1889, when 

 the experiments were stopped. 



The quantity of sewage passing through the filter in 

 the four months previous to June 18, 1888, was at the 

 rate of about 30,000 gallons per acre per day, except 

 during a part of March, when a larger quantity was 

 flowing. After June 18, 1888, through October, 1889, 

 the daily quantity was nearly double, averaging 58,000 

 gallons per acre per day. 



On August 21, 1888, a slight deposit of organic 

 matter was observed on the surface, but after a week 

 this disappeared, and then the only change apparent in 

 the surface, from the time when sewage was first ap- 

 plied, was the growth of some small tufts of grass. 

 Although the sewage was distributed over the whole 

 surface with care, it was found that in September and 

 October 1888 the greater part of the sewage was flow- 

 ing to spots on the surface which were lower than the 

 remaining area, and having become choked allowed 

 less sewage to pass, causing a delay in its disappearance. 

 Slight inequalities in the surface were removed by lightly 

 raking over about one-half the area of the tank, and 

 the tufts of grass were pulled up. A contrivance was 

 also arranged to ensure an exact distribution of the 

 sewage over the whole area, and the surface, with the 

 slight exceptions mentioned above, remained untouched 



