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64
The main facts which emerge as a result of the experiments already made are—
(1) The usual average percentage of dry matter in sludge sent to sea is
from 6 to 7. Cargoes containing as much as 10 per cent, of dry matter are
very unusual.
(2) An increase in the amount of dry matter in a sludge usually indicates
an increase in the amount of organic matter in sewage even under present
conditions where detritus and sludge are sedimented together.
(3) A comparison of the composition of the sludge sent to sea in a week
with the closely-preceding weekly average rainfall, volume of sewage treated,
or number of hours for which channels were in use before being emptied, discloses
no relationship.
It is hoped during the present year to conduct some large-scale experiments in
de-watering.
Greenwich
Power
Station.
Systematic chemical examinations have been carried out in connection with
the steam-raising plant at this station and a more effective control of its working
has been obtained. The coal and other fuel supplied to this and other stations and
premises belonging to the Council have been examined both for composition and
calorific value at the Greenwich Laboratory, which is specially equipped for this
work.
Investigations on special matters arising out of the working of the stations
are undertaken at this laboratory and, in part, at the laboratory at County Hall.
During the past year a considerable amount of work has been done in connection
with the steam generating plant at Greenwich.
Observations of Visibility and Actinicity.
The observations on visibility referred to in the reports for 1926 and 1927, were
continued during 1928. Daily observations were made at noon from the flêche
of the County Hall by the identification of a large number of well-defined objects
at varying distances in all directions and by noting in each case the distance from
the County Hall as measured on the map. By this means a closed figure can be
drawn representing the limit of visibility in all directions.
The marked seasonal variations in visibility have been commented on in previous
years. On page 59 of my report for 1926, the figures for spring, summer, autumn
and winter are shown, in which summer visibility is by far the greatest and winter
least, whilst the seasons around the equinoxes occupy intermediate positions. In
1927, evidence, based on observations on the lower Thames and in the open estuary,
was adduced showing that the low visibility of winter observed in London is not
common to the river valley. More systematic observations on the river during
1928 have fully confirmed the marked difference observed in 1927. These observations
have been made from the bridge of the sludge ship used as a laboratory by
the chemical assistant undertaking the examinations of the lower river.
The results obtained are summarised in the Table which represents the ratio
of the mean limit of visibility in all directions from the ship at the place named to
that found by simultaneous observation at County Hall. It will be seen that even
at places cn the eastern fringe of the County like Beckton and Crossness the visibility
from September to May was distinctly greater than at County Hall, whilst from the
Mucking Light (say, towards Canvey Island) downwards it was even more marked.
The visibility in summer tends to become uniformly good.
It seems reasonable to conclude that the explanation is that whereas a sea or
river fog is likely to affect the whole river valley, reducing visibility generally in
cold weather, in warmer weather the large and dry region of the more thickly populated
areas enables the droplets of condensed water which constitute such a fog to
be absorbed by the relatively hot air, and thus accounts for rather better summer
visibility in London. On the other hand, the great use of fuel in the London area