ay and night. The raking expedient introduced at this time
overcame this, and Mr. Hardy states that it is still followed when
the work is at all pressing. The speaker has found at Pittsburg, as
Mr. Hardy has found at Washington, that raking is nearly if not
quite as effective as scraping in restoring the filter capacity.

Eleven years ago the speaker was connected with the preliminary
investigations into the best methods of purifying the Potomac River
water for Washington. It then appeared that while for the greater
part of the time during an average year the Potomac River could be
classed among the clear waters of the East, there were periods when
excessive turbidity made it necessary to consider carefully methods
of preparatory treatment before this water could be filtered
effectively and economically. As Mr. Hardy has said, considerable
prejudice existed against the use of a coagulating chemical, and the
expedient was therefore adopted of giving the water a long period of
sedimentation in order to remove enough of the suspended matter to
allow the clarified water to be treated on slow sand filters. The
expert commission, consisting of Messrs. Hering, Fuller, and Hazen,
recommended the occasional use of a coagulating chemical, but this
recommendation was not carried out.

The Potomac River is somewhat peculiar, in that the turbidity of its
waters, as shown by the results presented in Mr. Hardy's paper,
ranges from 3,000 to practically nothing. The bacterial content also
varies widely, and Mr. Hardy's tables show this variation to be from
76,000 to 325 per cu. cm. Such a water as this requires particularly
careful preparatory treatment. The Dalecarlia Reservoir has a
capacity of something like 2 days' storage, the Georgetown Reservoir
the same, and the McMillan Park Reservoir nearly 3 days, making a
total sedimentation of more than 7 days. Without the use of a
coagulant, it is significant that during a period of five years,
even with 7 days' sedimentation, the average maximum turbidity of
the water delivered to the filters was 106 parts per million, and
the maximum average turbidity in one month was 250 parts per
million. The water filtration engineer can readily understand that
waters as turbid as this cannot be treated economically and
efficiently in slow sand filters. It would appear that coagulating
works might advantageously have been installed at the entrance to
the Dalecarlia Reservoir. If this had been done,