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92
purification. The only means adopted for that
purpose are storage, when some purification by
subsidence and the action of sunlight takes place,
and filtration through sand. The storage capacity
of the West Middlesex Company's reservoirs amounts
to 6'3 days' supply, and that of the Grand Junction
Company to 3-4. The larger the storage capacity,
the easier it is for the Company to avoid taking
water during floods, when there is always present a
larger proportion of impurities, both organic and
mineral.
The filter beds of the West Middlesex Company
have an average thickness of 2.6-ft., and those of
the Grand Junction, one of' 1.9-ft. The former
company filter water at the rate of 1.3 gallons per
hour per square foot of the filter, and the latter
company at the rate of 1.9 gallons. On a priori
grounds, therefore, the water supplied by the former
company should be better filtered.
The efficiency of the processes of purification are
measured (1) by the transparency of the filtrate;
(2) by the chemical parity of the same; and (3)
by the diminution observed in the number of microorganisms
after filtration.
The observations as to transparency are classified
under four heads. These, together with the results
of the examinations made last year, are given
below:—