| 20 | 42 | 0.81 | 49
December | 14 | 57 | 0.94 | 40
==========+=========+===========+===============+=============

The operation of this plant during the first year and a half offered an
excellent opportunity for the study of sedimentation in the sand, and
the data in Table 30 are presented to show that certain of the phenomena
of filter operation observed during this period seem to be fairly
explicable by the physical theory of purification. These data are given
only for the period of operation before the summer of 1907. At that time
the experiments in filter cleaning already described were begun. Before
that time, whenever a filter had been cleaned, all the discolored sand
had been removed, leaving for the following run a new sand surface
substantially in the perfect condition of a newly-constructed filter.
After that time the experimental methods of cleaning, and the new
routine adopted as a result thereof, interfered with the tracing of the
evidence as clearly as during the earlier periods.

[Illustration: ~Figure 14--Periods of Service and Depths of Scraping
for Runs Ending in Various Months Covering Entire Period Oct. 1, 1905,
to Mar. 1 1907.~]

Table 30 and the corresponding diagram, Figure 14, show the general
variations in the length of runs and depth of penetration, with the
seasonal temperature changes. The increase in length of runs and
quantity of sand removed under low temperature conditions is very
marked. There is, however, a secondary maximum which appears, as the
diagram shows, where a minimum for the year would be expected. This
may have been an irregularity occurring this one year, which will
not appear in the average of several years, and caused by some
factor which has escaped observation. A careful analysis of the data
at hand fails to show any explanation for it. It may exist in some
of the little-understood biological actions which have their maximum
effect under warm-water conditions, or it may be due--in some
obscure way--to the liberation of air under the surface of the sand,
accumulating with pressure enough to break the surface at
innumerable points, thereby reducing the loss of head and extending
the period of service. Some evidence was observed pointing to this
explanation, but it was never conclusively proven.

The general effect of temperature changes on the rapidity of removal
of the sediment and its consequent concentrat