due, in part, to the excellent method
of manipulation described in the paper. It is his feeling, however,
that part of the success is attributable to the very low uniformity
coefficient of the sand. In other words, the sand grains are nearly
all of the same size, due to the character of the stock from which
the filter sand was prepared; and, therefore, there is much less
opportunity for separation of the sand according to grain sizes than
there would be with the filter sand which has been available in most
other cases. Filter sand with a uniformity coefficient as low as
that obtained at Washington has been rarely available for the
construction of sand filters, and while the method of hydraulic
return should certainly be considered, it will not be safe to assume
that equally favorable results may be obtained with it with sands of
high uniformity coefficients until actual favorable experience is
obtained.

The writer believes that in calculating the cost of the water used
in the plant itself the price chosen by the author, covering only
the actual operating expenses of pumping and filtering, is too low.
The capacity of the whole Washington Aqueduct system is reduced by
whatever quantity is used in this way, and, in calculating the cost
of sand handling, the value of the water used should be calculated
on a basis which will cover the whole cost of the water, including
all capital charges, depreciation, operating expenses, and all costs
of every description. On this basis the water used in the
sand-handling operations would probably be worth five or more times
the sum mentioned by the author.

The cost of operation of the plant has come within the estimates
made in advance, and has certainly been most reasonable. The cost of
filter operations has averaged only about 50 cents per million
gallons, and is so low that it is obvious that the savings which may
be made by introducing further labor-saving appliances would be
relatively small. It will be remembered that ten or fifteen years
ago the cost of operating such filters under American conditions was
commonly from $2 to $5 per million gallons.

The experiments represented by Tables 17 to 19, inclusive, serve to
show that preliminary filtration, or multiple filtration, or any
system of mechanical separation is incapable of entirely removing
the finer clay particles which cause the residual turbidity in the
effluent. They also show that this turbidity may be easily and
c