of rate on efficiency.

[Footnote 1: _Journal_, New England Water-Works Association, Vol.
24, p. 589.]

The experiments show that turbidity passes more freely at the higher
rates with the Potomac water, as has also been found to be the case
with other clay-bearing waters.

In the last lines of Table 20 are given cost per million gallons for
filtering at various rates. There is no discussion of these figures,
and as they differ considerably from those which the writer has been
accustomed to use, the calculation in Table 23, made three years ago
for a particular case, may be of interest.

~Table 23--Relative Cost of Filtering at Different Rates.~
======================+===================================================
|Nominal rate, in millions of gallons per acre daily:
+------------+------------+------------+------------
| 3 | 5 | 10 | 20
----------------------+------------+------------+------------+------------
Percentage which | | | |
average yield is of | | | |
nominal rate..........| 85 | 80 | 75 | 65
----------------------+------------+------------+------------+------------
Average output per | | | |
acre, in millions of | | | |
gallons per day.......| 2.55 | 4.00 | 7.5 | 13.0
----------------------+------------+------------+------------+------------
Cost of that part of | | | |
filters per acre | | | |
dependent on rate.....| $12,000 | $20,000 | $40,000 | $80,000
----------------------+------------+------------+------------+------------
Cost of that part of | | | |
filters per acre not | | | |
dependent on rate.....| 50,000 | 50,000 | 50,000 | 50,000
----------------------+------------+------------+------------+------------
Total cost of filters | | | |
per acre..............| 60,000 | 70,000 | 90,000 | 130,000
----------------------+------------+------------+------------+------------
Cost per million | | | |
gallo