ve been clearly demonstrated. Its practical
effect is to allow a greater quantity of water to be passed between
scrapings, thereby saturating--if the term may be used--the surface
layer with clay and other fine matter before removing it, instead of
taking it off when only a thin surface layer of it has been thus
saturated.

The large proportion of the total purification that takes place in
passing through three reservoirs successively, holding in the
aggregate a quantity of water equal to about 7 days' use, is very
striking. Taking all the records, the percentage remaining after
passing through these reservoirs, is as follows:

Sediment for the year, 1909-1910, Table 2....................17%
Turbidities, 5-year average, Table 3.........................25%
Bacteria, 5-year average, Table 4............................24%
Bacteria, selected winter months with high
numbers in the raw water...................................20%
Bacteria, selected summer months with high
numbers in the raw water................................... 2.5%

There is considerable seasonal fluctuation in the results of
settling and filtering, as is shown in Table 21.

~Table 21--Average Removal of Turbidity and Bacteria by
Washington Filters for Whole Period, Arranged by Seasons~.
===========================+========+========+========+========+========
| Winter.| Spring.| Summer.| Fall. | Year.
----------------+----------+--------+--------+--------+--------+--------
Turbidity, in | raw | 135 | 96 | 144 | 42 | 105
parts per | settled | 33 | 28 | 27 | 15 | 26
million: | filtered | 4 | 3 | 1 | 0.5 | 2
----------------+----------+--------+--------+--------+--------+--------
Percentage | settling | 24 | 29 | 19 | 36 | 25
left from: | filtering| 12 | 10 | 4 | 3 | 8
| both | 3 | 1 | 0.3 | 1 | 2
----------------+----------+--------+--------+--------+--------+--------
Bacteria per | raw | 16,600 | 4,150 | 4,100 | 1,960 | 6,700
cubic | settled | 6,300 | 980 | 160 | 270 | 1,940
centimeter: | filtered | 149 | 29 | 18 | 22 | 54
----------------+----------+--------+--------+--------+--------+--------
Percentage | sett