t covered by the author's experiments, and of many hundreds of
bacterial analyses of each effluent, and form, with the author's
experiments, the most thorough-going studies of the effect of rate
on efficiency that have come to the writer's attention.

Mr. Clark's results are given in Table 22.

~Table 22.~
===========+============+===========+=============+===========+============
| | | | | _B. Coli_
| | | | |in 1 cu.cm.
Effective | | Rate | Bacteria per| |(percentage
size of | | in gallons| cubic | Bacterial |of positive
sand. | Filter No. |acre daily.|centimeter in|efficiency.| tests).
-----------+------------+-----------+-------------+-----------+------------
0.28 | A | 3,000,000 | 48 | 99.1 | 5.0
0.25 | B | 5,000,000 | 85 | 98.4 | 24.0
0.22 | C | 7,500,000 | 105 | 98.1 | 25.0
0.22 | D |10,000,000 | 110 | 98.0 | 25.0
0.22 | E |16,000,000 | 280 | 95.0 | 38.0
===========+============+===========+=============+===========+============

It will be seen that the number of bacteria passing increases
rapidly with the rate, and whether the total number of bacteria is
considered or the _B. coli_ results, the number passing is
approximately in proportion to the rate. In other words, doubling
the rate substantially doubles the number of bacteria in the
effluent.

This is entirely in harmony with all the Lawrence experimental
results extending over a period of 20 years. There have been
occasional apparent exceptions, but, on the whole, experience with
Merrimac River water has uniformly been that more bacteria pass as
the rates are higher.

The theory sometimes advanced, that the efficiency of filtration is
controlled to a certain extent by gelatinous films, and that, as far
as thus controlled, is less dependent on rate, would not seem to be
borne out by these results. The Merrimac River water, carrying large
amounts of organic matter, would certainly seem better adapted to
the formation of such films than the clay-bearing Potomac water,
comparatively free from organic matter; but it is the Potomac water
which seems to show the least influence