assume that about
one-half of the positive results, would have been eliminated had the
confirmatory steps been taken. In other words, the figures showing
the number of positive tests for _B. Coli_ in Table 20 should be
divided by two when comparing them with corresponding ones for the
main filters.

[Footnote 1: Published in the _Journal of Infectious Diseases_,
Vol. 4, No. 3, June, 1907.]

Mr. Knowles seems somewhat apprehensive regarding the methods described
in the paper of restoring the capacity of the filters by raking, and
replacing sand by the hydraulic method, and yet, from Mr. Johnson's
discussion, it would seem that the practice of raking filters between
scrapings had recently been adopted at the Pittsburg plant.

Before the practice of raking was finally adopted as a part of the
routine filter operation, the subject was given a great deal of
thought and study, as may be seen by referring to Mr. Longley's
discussion.

The re-sanding has been done by the hydraulic method, for nearly two
years, and, as far as the writer is able to judge, this method has
been more economical and also more satisfactory in every way than
the old one. As Mr. Hazen states, this does not prove that the
hydraulic method would be as satisfactory for other filter plants
and other grades of sand. The elevated sand bins at the Washington
plant fit in well with this scheme, and save the expense of one
shoveling of the sand; and the low uniformity coefficient of the
sand is favorable in decreasing its tendency to separate into
pockets or strata of coarse and fine sand. The method of washing is
also well adapted to this method of re-sanding, as the sand is made
very clean in its passage through the washers and storage bins. The
hydraulic method of replacing sand tends to make it cleaner still,
because any clay which may be left in the sand is constantly being
carried away over the weir and out of the bed, to the sewer. Sand
replaced by the hydraulic method is much more compact than when
replaced by other methods, and consequently the depth of penetration
of mud in a filter thus re-sanded is less. Careful tests of the
effluents from filters which have been re-sanded by the two methods
have invariably shown the superiority of the hydraulic method.

The experiment of replacing sand by water, referred to by Mr.
Longley, was not considered a success at the time, and the method
was abandoned for about a year. At that time an attempt wa