he rate is greater--and, second, those coming from harmless
growths in the under-drains and lower parts of the filter--the
numbers of which per cubic centimeter are presumably less as the
rate is greater--and these two parts, varying in opposite
directions, may balance each other, as they seem to do in this case,
through a considerable range. It may thus be that the number of
bacteria really passing the filter varies much more with the rate
than is indicated by the gross results.

It is also of interest to note that the sand filter (called a
preliminary filter) in Table 18, filled with the same kind of sand,
when operated at an average rate of 50,000,000 gal. per acre daily
for a year, allowed 18% of the applied bacteria to pass, in
comparison with 3% found in Filter No. 6 of Table 20, operated at an
average rate of 38,000,000 gal. per acre daily.

There was one point of difference in the manipulation: the
preliminary filter was washed by a reversed current of water, as
mechanical filters are washed, while Filter No. 6 was cleaned by
scraping off the surface layer, as is usual with sand filters.
Whether the great difference in bacterial results with a relatively
small difference in rate is to be attributed to this difference in
manipulation the writer will not undertake to state.

If the experimental results of Table 20 indicate correctly the
conditions which obtain in filtering Potomac water, then increasing
the rate of filtration so as to double it, or more than double it,
would make but little difference in the quality of the effluent as
measured by the usual bacterial methods. If the increase in rate
were accompanied by the preliminary filtration of the water, then,
presumably, there would be little change in the quality of the
effluent, and the maintenance of excellent results might be
incorrectly attributed to the influence of the preliminary filter.

It would also seem that the apparatus which is sometimes used for
determining and controlling the rate with more than the ordinary
degree of precision is hardly justified by such experimental results
as those presented by the author.

In contrast to these results may be mentioned those obtained by Mr.
H. W. Clark,[1] for experimental filters operated with Merrimac
River water, at rates ranging from 3,000,000 to 16,000,000 gal. per
acre daily. The results are the average of nearly two years of
experimental work, the period having been nearly coincident with
tha