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49
extent. Besides contamination due to bathers, sunlight acting on bicarbonate waters
of the kind supplied to London will lead to the development and growth of myriads
of microscopic green algæ which themselves colour the water, and which cause large
amounts of insoluble carbonate of lime to separate as mud, causing turbidity or
sediment, according to the amount to which the bath is being used. The turbidity
soon becomes sufficient to make it impossible to see anything or anyone much under
the surface of the water. Filtration, combined with the use of coagulants and bleaching
powder, intelligently used, has a markedly beneficial result so far as the physical,
biological and chemical character of the water is concerned.
It has been found that very frequent renewal of water in baths not fitted with
filters may be avoided by adding copper sulphate—less than one part per million—
to stop the growth of algae and subsequent chlorination to sterilise or partially
sterilise the water. Although this treatment is better than none at all, and cheaper
than very frequent renewal of water, it cannot be considered as more than a palliative.
In the course of this work it was sought to ascertain the average contamination
introduced by a bather by comparative analyses before and after use of the water of
a small swimming bath at a school. Having regard to the respective weights and
surfaces of the children using the bath and of adults it would appear that a not very
clean adult would contribute while in the bath about 0.8 gramme (say 12 grains) of
nitrogen in all forms and 1.4 grammes of chlorions. It is probable that the preliminary
use of a shower bath and other precautions would reduce this amount appreciably.
In last year's report there was some account and discussion of observations made
in the years 1923-6 of the horizontal range of visibility in all directions from the
Fleche at County Hall. It was noted that while the seasonal changes in visibility
and actinicity are due largely to natural conditions lying wholly outside human
control, there can be no question that coal smoke is an important and aggravating
factor in screening actinic light and intensifying the obscuration to which London
is subject by reason of its situation in the valley of a large river. Early in 1927
comparison showed that the range of visibility in the Thames estuary was very
much greater than from the County Hall on the same day. A series of observations
was therefore made from the sludge ship used as a laboratory for river examination,
of the most distant objects visible in all directions at midday. As the movements of
the ships are regulated by tide it is impossible to obtain regular daily observations
at any one place, but by selecting a few places whence well-defined distant
objects can be seen under good conditions a fair number of continued observations
were made. Those taken off Southend have been chosen as representing a place
well away from London yet fully subject to meteorological variations due to the
river itself.
When observations taken at approximately the same time of day at Southend
and County Hall are compared it is seen that at the former place the areas of
visibility are approximately circles centred on the place of observation. The County
Hall diagrams show in most cases not only a smaller range of visibility but a lessened
visibility in the direction towards which the wind is blowing. The summer results
so far as can be judged do not suggest the probability of any great systematic difference,
but from October to December it is evident that a cause which has reduced
the visibility at County Hall to a mile or less (rather more in October) has had very
little effect on the Southend results, which show as good visibility on some days
in December (8-10 miles) as in the summer. It may be noted that on December 28th,
after the great snowstorm of Christmas, Ashdown Forest was clearly visible from
Co'dharbour, more than 25 miles away, at midday, whilst the average range of
visibility at County Hall was only a mile. It is remarkable that the actinic value
as measured by Watkin's "Bee" meter was not usually higher at places down
the river than at County Hall, notwithstanding the materially wider range of
visibility.
Observations
of visibility
and
actinicity.