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249
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 supplied by the Grand Junction,
West Middlesex, and Chelsea Companies, respectively, last
year, averaged, in round numbers, 35, 29, and 37 gallons daily
per head of the population, and 251, 173, and 233 gallons per
house. The "intake" of the Chelsea Company is at West
Molesey, the intakes of the other Companies at Hampton.
The Grand Junction supply is supplemented from sand and
gravel beds at Hampton, the Company having a complete
system of underground collection of water on about 20 acres of
land, and being able, during the time of flood, to raise large
quantities of water that has passed through the natural beds of
gravel and sand, which are characteristic of the neighbourhood.
Owing to the scanty rainfall, and the infrequency of freshets
and floods in the rivers, the conditions under which filtration of
the Metropolitan water supply was carried on throughout the
year were exceptionally favourable, and the results, as regards
freedom from turbidity, and low tone of colour in the water,
were generally satisfactory. It need hardly be added that
polluted water is very difficult to filter. The solid impurities
in suspension, chiefly consisting of clay, marl, and chalk, in a
very finely divided state, can be got rid of only by long subsidence
previous to filtration.
The fact that the impurity of the water in the river is so
largely increased during periods of flood, renders it of great
importance that there should be sufficient provision of subsidence
and storage reservoirs to enable the necessary daily supplies to
be delivered during such periods, while keeping the intakes
closed. The position of the several Companies varies in this
respect very considerably. The following statement shows the
number of days' supply, represented by the capacity of the
reservoirs for unfiltered water, belonging to each of the local