the plant consisted of cylindrical galvanized-iron
tanks 4 ft. in diameter and 9 ft. high. The filter sand in these
tanks was taken from the supply for the main filters. It was
supported on gravel layers and supplied with under-drains of
suitable sizes for the proposed rate of flow in each case.

The units of the experimental plant were designated as Nos. 1, 2, 3,
4, 5, and 6, and it was the original intention to operate them at
rates of 1,000,000, 3,000,000, 6,000,000, 10,000,000, 30,000,000,
and 100,000,000 gal. per acre daily, respectively.

This schedule of rates was carried out in a general way with all the
filters, with the exception of Nos. 5 and 6. For these, the rates
were found to be higher than could be maintained for any great
length of time, owing to the deeper penetration of the mud in the
filter sand, which caused high initial losses of head, short runs,
and deep scrapings. A rate of about 30,000,000 gal. was maintained
in the case of Filter No. 5 from the time it was started on
September 9th, 1908, until November 8th, 1909, when it was reduced
to about 17,000,000 gal., which rate was maintained thereafter until
the filter was shut down in February, 1910.

[Illustration: ~Figure 7--Plan of Filter-House and Equipment.~]

In the case of Filter No. 6, it was found impossible to maintain a
rate of 100,000,000 gal. for more than a very few days at a time. It
was started at about this rate, however, at the beginning of each
run, and kept as high as possible for the remainder of the time
during the first seven runs. At the end of the seventh run, on
October 17th, 1908, the filter was given a very deep scraping and
re-sanded.

[Illustration: ~Figure 8--Diagrammatic Sketch Showing Arrangements for
Testing "Puech" System of Water Filtration at Washington D. C.,
U. S. A.~]

The layer of clean sand restored the original capacity, and the
filter was operated as before, but with gradually decreasing rates
until December, 1908, when the rate was reduced to about 40,000,000
gal. Even this lower rate was too high to be maintained without
removing and replacing a large part of the sand. The rates,
therefore, gradually decreased to about 23,000,000 gal. on March
13th, 1909, when the filter was again re-sanded. After this
re-sanding the rate was reduced to about 20,000,000 gal., and the
filter was operated at approximately that rate until it was again
re-sanded on November 13th, 1909, when the rate was again reduced