owing feeling that the water supply, before filtration
was introduced, had been blamed for more than its share of the
typhoid, and this is borne out by much evidence that has been
presented from time to time.

It is not an unreasonable conjecture, therefore, that perhaps the
reduction of one-third in the total typhoid death rate may represent
a much larger reduction in that part of the total which was due to
polluted water alone; and that, as the authorities in the District
of Columbia and in certain other cities, particularly in the South,
are now recognizing, the fight against much of the remaining typhoid
must be in the direction of the improvement of milk supplies,
precautions against secondary infection, and attention to a large
number of details surrounding the individual, which may effectively
protect him against the insidious attack of the disease favored by
unknown agencies.

~Experiments in Filter Cleaning~.

The author refers to the difficulty encountered during the first two
summers in keeping the filters cleaned fast enough to maintain the
capacity of the plant. The real seriousness of this may be judged
from the following facts. The average increase in loss of head on
all the filters for the entire year, July 1st, 1906, to July 1st,
1907, was about 0.053 ft. per day. During the 1906 period of low
capacity under discussion, the loss of head on twelve of the filters
increased for a period of eight days at the average rate of 0.45 ft.
per day, or about nine times the normal rate of increase. This
difficulty was caused by the presence of large numbers of
micro-organisms in the applied water. During the first summer (1906)
this fact was not recognized, but the sudden decrease in capacity
was supposed to have been caused by the unusually high and
long-continued turbidity which prevailed during that summer in the
Potomac River, and persisted in the water supplied to the filters
even after about four days of sedimentation in the reservoirs.
During the second summer (1907) the same phenomenon of suddenly and
rapidly increasing losses of head appeared again, but without any
unusual turbidity in the applied water. Investigation, however,
showed the presence of large quantities of organisms, particularly
_melosira_ and _synedra_, in the applied water, and examinations in
subsequent years have shown a periodic recurrence of these forms in
quantities sufficient to cause the trouble mentioned. In June, 1907,
exami