ere economy
of operation; the economy of human life, the effect on which
requires far longer than a few months of trial to determine, is a
much more important factor. Believing that no one should depart,
until after a long period of conclusive experimentation, from that
principle which is known to be safe (viz., to take off a small
portion of the clogging surface), the writer studied to determine
more efficient and economical methods of accomplishing this end.

A device for scraping the material, in just the same way as with
shovels, but more efficiently and more exactly, was developed by
George P. Baldwin, M. Am. Soc. C. E., under the general supervision
of the Bureau of Filtration, of which the writer was in charge.
However, on account of the unfortunate and earlier arrangement of
other constructive matters, which the City's Legal Department
advised could not be changed without upsetting the contract, the
entrance doors to the original forty-six filters were not built
large enough to permit the rapid and economical transfer of these
machines, and, as this act takes so large a proportion of the total
time of operation, it has not been found economical to use them. The
additional ten filters, recently constructed, with doors especially
designed and large enough to pass the machines, have not yet been
placed in operation. This is said to be on account of lack of funds
and of employees. Therefore, there has been no opportunity to
demonstrate what the scraping machines can do, under the conditions
for which they were designed to operate. The restoring machine, a
complementary device in mechanical operation, which simply replaces
the sand in the same condition that it would be if wheeled back,
but, with a small percentage of moisture, has accomplished its
purpose well and economically. The sand is placed in the filters so
that there is no further settling; with a smooth surface, needing no
additional adjustment; with absolutely no possibility of sub-surface
clogging; and with the filters starting off exceedingly well in
operative results.

_Washington Methods._--In Washington, it is stated that the filters
are still cleaned by the old-fashioned method of scraping with
shovels, throwing the sand into piles, and afterward removing it
with a movable ejector. Between scrapings there is also an
occasional mid-period action of raking the unwatered sand surface,
for the purpose of stirring up the dirty film. This process does n