ed or partly-settled water and supplying from
the main currents water containing more suspended matter particles
to be removed. There is thus a considerable percentage of the total
volume of voids in which the water is subjected to very favorable
conditions for sedimentation, almost perfect stillness and an
exceedingly small distance for a particle to settle before it
strikes bottom on the surface of a grain of sand.

If sedimentation were the predominating factor in the purification
of the water, we would then expect to find the following phenomena
in the operation of the filters: A more rapid deposition of a given
amount of sediment under summer temperature conditions than under
winter, as the water passes through the sand, and therefore, for the
former condition of higher temperature:

(a) A greater concentration of this turbidity-producing material
in the top layer of sand, or, in other words, a thinner sand
layer to be removed in scraping if all the dirty sand is
removed;

(b) Because of the greater concentration, a greater rate of
Increase of the loss of head, and consequently shorter periods
of service between scrapings;

(c) A higher limit for turbidity in the water applied to the
filter to produce a given turbidity in the effluent.

~Table 30--Service Periods and Scraping Depths for Runs
Ending In Various Months; Covering Entire Period,
October 1st, 1905, To March 1st, 1907.~
==========+=========+===========+===============+=============
| | Average | Average | Mean
| Number | period of | depth of sand | temperature,
Month. | of | service | removed, in | in degrees,
| filters.| in days. | inches. | Fahrenheit.
----------+---------+-----------+---------------+-------------
January | 13 | 75 | 2.09 | 39
February | 6 | 98 | 2.46 | 37
March | 5 | 130 | 2.66 | 41
April | 8 | 149 | 2.96 | 53
May | 7 | 130 | 2.80 | 67
June | 11 | 124 | 2.35 | 77
July | 17 | 70 | 2.12 | 81
August | 2 | 49 | 1.98 | 80
September | 5 | 73 | 2.48 | 76
October | 37 | 70 | 1.56 | 64
November