ot
remove any of the clogging material from the bed, but it is said
that no injurious effects are produced, and that it is economical.
It is stated that the so-called "Brooklyn method," of stirring the
surface of the sand while the water is on the bed, has been tried at
Washington, but with unsatisfactory results. It seems to have been
advocated with greater fervor in some other places.

The method of dry raking does not remove the dirty material, but
loosens up the pores of the surface, and through this porosity
permits clogging to penetrate deeper into the filter. The method of
raking with water on the bed, although it removes some of the
organic dirt, also permits deeper penetration of the remainder. The
latest devised system of washing the sand in place, by upward
spraying with water, called the "Blaisdell method," thoroughly
destroys the _Schmutzdecke_ above, and, at the same time, must
permit the formation of a subsidiary one below. In the Nichols
method, the material removed by shovel scraping is conveyed by an
ejector to a portable separator, where it receives a single washing;
the dirty water overflows to the sewer, while the washed sand is
discharged through a hose and deposited on the recently scraped
surface. As the latter is partly impregnated with impurities, there
is, by this process, a tendency toward sub-surface clogging.

All these processes are marked and serious departures from the
well-tried method of cleaning slow sand filters, which, it is well
known, will operate successfully to purify polluted river waters and
make them safe to drink. In all there is the danger that they have
not been sufficiently and carefully tried, under scientific
observation, as to results and possible effects on the public
health, to be sure that the bacterial efficiency can long continue
to be satisfactory, with the application of specifically infected
waters. It is dangerous, and may even jeopardize the safety of human
lives, to experiment on water which is furnished for drinking
purposes. There is also the added danger, well known from past
experience, that in a few years (it may be more or less, depending
on the extent and intensity of the new workings) the filters will
need renovation, partly, if not wholly, throughout the entire bed.
Thus, considering the total cost during a long term of years, the
apparently cheaper method may become the most expensive.

There is also an interesting query in regard to the Washin