to separate into different sizes, and it is fortunate if this work
can be done in such a manner as to avoid this separation entirely.

It may be questioned whether a certain amount of segregation of the
materials will make any practical difference in the efficiency of a
filter. In all probability this depends on the degree of the
segregation, the quantity of pollution in the water to be filtered,
the rate of filtration, and the uniformity of methods followed in
the operation, etc. For an applied water as excellent in quality as
that of the Washington City Reservoir during favorable summer
conditions, a considerable degree of segregation might exist without
producing any diminution in efficiency. For a badly polluted water,
however, such as the applied water at this plant during certain
winter periods, or the water of a great many other polluted
supplies, it might be found that even a slight lack of homogeneity
in the sand might make an appreciable difference in the results of
filtration.

As a result of the experiments herein described, however, this
method may be applied at other plants where conditions seem to
warrant it, with a largely increased measure of confidence;
although, as in the case of the adoption of any new or radical
departure, that confidence must not be permitted to foster contempt
of the old and tried methods, but its operation must be watched with
the utmost caution, until long experience shall have demonstrated
its perfect suitability and defined its limitations.

~E. D. Hardy, M. Am. Soc. C. E.~ (by letter).--It was not the
writer's original intention to enter into a discussion of either the
theory of water purification or of the experimental work on sand
handling, but simply to present the main results of operation
largely in tabular form. He is gratified, however, to have these
sides of the question so ably brought out in Mr. Longley's
discussion.

Mr. Hazen referred to the inferior efficiencies of the experimental
filters for rate studies (as shown in Table 20) in the removal of the
_B. Coli_ from the water tested. This inferiority is really less than
the figures in the table would indicate, as the tests for the
experimental filters were presumptive only (as shown by the note at the
foot of Table 20), while those for the main filters were carried through
all the confirmatory steps.

From experiments[1] made by Messrs. Longley and Baton in the
writer's office, it would seem reasonable to