valves for carrying water,
controlling rates of filtration, etc.

[Footnote 1: Presented at the meeting of February 15th, 1911.]

[Footnote 2: "Works for the Purification of the Water Supply of
Washington, D. C.," by Allen Hazen and E. D. Hardy, Members, Am.
Soc. C. E., _Transactions_, Am. Soc. C. E., Vol. LVII, p. 307.]

In the pumping station, there are three centrifugal pumps, which are
directly connected to tandem compound engines; two sand-washer
pumps; three small electric generating sets for furnishing electric
light; and four 200-h.p., water-tube boilers.

Each of the centrifugal pumps has a nominal capacity of 40,000,000
gal. per day when pumping against a head of 21 ft., and each
sand-washer pump has a capacity of 2,500,000 gal. when pumping
against a head of 250 ft. The electric light engines and generators
supply the current for lighting the pumping station, the office and
laboratory and other buildings, and also the courts and interior of
the filter beds, and for operating a machine-shop.

The filters and filtered-water reservoir are built entirely of
concrete masonry. The floors are of inverted groined arches on which
rest the piers for supporting the groined arch vaulting. All this
concrete work is similar to that in the Albany, Philadelphia, and
Pittsburg filters.

The filters contain, on an average, 40 in. of filter sand and 12 in.
of filter gravel. The gravel is graded from coarse to fine; the
lower and coarser part acts as part of the under-drain system, and
the upper and finest layer supports the filter sand. The raw water
from the pumps is carried to the filters through riveted steel
rising mains which have 20-in. cast-iron branches for supplying the
individual filters. The filtered water is collected in the
under-drainage system of the several filter beds, and is carried
through 20-in., cast-iron pipes to the regulator-houses. These
regulator-houses contain the necessary valves, registering
apparatus, etc., for regulating the rate of filtration, showing the
loss of head, shutting down a filter, filling a filter with filtered
water from the under-drains, and for turning the water back into the
raw-water reservoir, or wasting it into the sewer. From the
regulator-houses, the filtered water flows directly to the
filtered-water reservoir. Generally, five filters are controlled
from one house, but there are two cases where the regulator-houses
are smaller, and only two filters are cont