BY HAXDOLPH WASTENEYS. 119 
order, and secondly—Even when matured, their effluents 
though bright and sparkling and in every other way desire- 
able, always contain a slight amount of matter in suspension, 
also as might be expected the reduction in the numbers 
of bacteria is comparatively poor. 
The consideration of the foregoing circumstances led to the 
conclusion that a combination consisting of a bed of this type 
worked in conjunction with an ordinary sand filter would 
give excellent results ; the rapid passage of the water through 
the gravel bed would it was thought effect a partial purifica_ 
tion of the water without removing all the suspended matter, 
at the same time aerating it, and thus rendering it in every 
way more amenable to treatment by the sand filter. It 
was decided to try the experiment. A small square gravel 
filter of the Morry type was constructed with an effective 
area of 12 square feet and a depth of 3 feet, a small sand 
filter was also constructed, having an area of 18 square 
feet, and composed of 2 feet of fine sand. These two 
filters were arranged so that the sand filter might be con- 
veniently fed with the effluent from the Morry filter. 
The sand filter was worked in the usual way, but with a 
depth of only 11 inches of water over the sand. Although 
only a poor quality of sand was used in this filter, the results 
of the analyses of its effluent have fully realised the expecta- 
tions before mentioned. When the combination was worked 
at a rate which gave a yield of water from the combined 
areas equal to that obtained from a similar area of the ordinary 
sand filters, the results obtained showed a _ reduction of 
organic matter, which was a distinct improvement on the 
results obtained from the best of the sand filters, and whilst 
the reduction of the numbers of bacteria was equally good. 
the amount of dissolved oxygen in the effluent averaged 
66 per cent. of the possible quantity during a period in which 
the oxygen in the ordinary sand filter effluents averaged 
only 10 per cent. 
Having considered the results obtained by filtration 
of the water without the assistance of coagulants, the next 
method for consideration is that of Mechanical Filtration. 
The main object of mechanical filtration is to purify 
large volumes of water with a small amount of filtering 
material. In order to effectively do this the addition of a 
