water could be
maintained continually through the winter as good as, or better
than, it is during the summer, then the filtered water would be of
the perfect sanitary quality desired throughout the entire year.

This was all foreseen ten years ago, when Messrs. Hering, Fuller,
and Hazen recommended auxiliary works for preliminary treatment of
the supply, although, as the author states, these works were not
provided for in the original construction. As prejudice against the
use of a coagulant seemed to be at the bottom of the opposition to
the preliminary treatment, a campaign of education bearing on this
point was instituted, in addition to the systematic studies of
different preliminary methods to which the author refers. As a
result of the combined efforts of all those interested in promoting
this improvement, an appropriation was finally made for the work in
1910. The coagulating plant has since been built, and the writer is
informed that coagulation was tried on a working scale a short time
ago during a period of high turbidity. A statement of the results of
this treatment on the purification of the water in the reservoir
system and in the filter plant would be of great interest.

[Illustration: ~Figure 15-- Turbidity in Applied Water.~]

_Hydraulic Replacing of Filter Sand._--The author has adopted a
method of replacing clean sand in the filters which will commend
itself to engineers as containing possibilities of economy in
operation. The first experiments in the development of this method
at the Washington plant were carried out some three years ago, while
the writer was still there. Substantially the same methods were used
then as are described in this paper, but examination of the sand
layer by cutting vertically downward through it after re-sanding in
this manner showed such a persistent tendency toward the segregation
of the coarse material as to hold out rather discouraging promises
of success. The greatest degree of separation seemed to be caused by
the wash of the stream discharging sand on the surface. It was
observed that, near the point where the velocity of the stream was
practically destroyed, there seemed to be a tendency to scour away
the fine sand and leave the coarse material by itself, and pockets
of this kind were found at many points throughout the sand layer.
The author states that, in the recent treatment of the filters by
this method, there has been no apparent tendency for the materials