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Grand Junction.—Harwich sand, 2ft. 6in. : Hoggin, 6in.; fine gravel, 9in. ;
coarse gravel, 9in.; boulders, 1ft, Total thickness. 5§ft. Another
and more modern kind of filter used by this Company is made up of
sand, 2ft.. tine gravel, 9in., pipe drains, 3in. Total 3ft.
Chelsea.—Thames sand, 4ft. 6in.: shells, &c., 3in. ; gravel, 3ft. 3in. Total
thickness, 8ft.
Sand, it may be added, varies in quality, the Harwich
sand, which is remarkably fine and uniform in grain, being the
best. The sand becomes clogged in course of time with deposited
mud and vegetable matter, and then the upper layer is scraped oft'
and removed. Renewals of sand should take place periodically
as the layer becomes reduced below an assigned minimum.
But filtration avails little if the water is allowed, as frequently
happens, to be fouled in the cisterns ; whether through the uncleanly
condition of those receptacles, or by the connection of
them with drains through the medium of waste-pipes. The
attention of householders cannot be too frequently drawn to the
necessity for the abolition of waste-pipes, and the periodical cleansing
of cisterns used for storing potable water. Cisterns, moreover,
should be properly covered so as to exclude light and dirt;
and so placed that the water may be kept cool in summer, without
being in danger of freezing in winter. When " constant
supply" is given, a screw-down draw-oft'tap should be affixed to
the rising main, or service pipe, so that drinking and cooking
water may be drawn therefrom, fresh and uncontaminated, without
the intervention of cisterns, which are required for washing,
flushing, baths, and similar purposes. A stop-valve should be
fixed outside the premises to prevent damage to the house in
case of pipes burst by frost or other causes.
The quantity of water for domestic purposes supplied by
the Grand Junction, the West Middlesex, and the Chelsea
Companies respectively, last year, averaged 27'8I ; 22'74 ; and
27"50 gallons, daily, per head of estimated population; and 250,
170, and 216 gallons per house. The "intake" of the Chelsea