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followed by very heavy snow. It was impossible to say how many of the "takes" recorded as
such were possibly washed away, but from a wide experience of sewer control under all weather
conditions it is reasonable to assume that approximately 15% were in fact washed away.
Accepting the recorded "takes" of the final test as all taken by rats, and not washed away,
the progressive rate of the complete "takes" of bait is 15, 58 and 194. The comparable results
of the tests applied prior to the series was 36, 81 and 165 complete "takes".
It is too early to express definite views on the efficacy of '1081' and similar controls will
continue throughout 1958 and 1959 but, from observations of the controls already carried out,
and comparison of the detailed information collated, the indications are that it might well prove
comparable to '1080'.
Bait acceptance of '1081' is extremely good and this is probably due to the slowness of the
onset of warning symptoms. It also has the advantage by reason of its coarser division of being
safer to dispense, and owing to lower toxicity to rats, and presumably to man, is less hazardous
in distribution.
The information gained from the test bait controls illustrates the rapid build-up and distribution
that occurs in a period of 6 months, and tends to emphasise the necessity of quarterly poison
controls, at least as far as a built-up area such as the City is concerned.
ATMOSPHERIC POLLUTION
The year 1957 was the second complete year of the coming into operation of the smoke
abatement provisions of the City of London (Various Powers) Act, 1954 and the first complete
year of certain of the provisions of the Clean Air Act, 1956. This Act will not become fully
operative until 1958 when railway locomotives — which were exempt from the provisions of the
City's Act — will become subject to Regulations to be made under the Clean Air Act specifying
the periods when emissions of smoke of prescribed densities, determined by reference to the
Ringelmann Chart, will be permitted.
No application under the Various Powers Act for approval of installations in existing buildings
burning a fuel other than an authorised fuel was received during the year. In compliance
with the Clean Air Act three notices of intention were given that furnaces in existing buildings
were to be installed or converted.
The number of registered complaints of smoke emissions was 27. These were mainly of a
temporary character due to mechanical faults, the burning of refuse — particularly on building
sites — and initial difficulties associated with the first lighting up of installations in new buildings.
In no case was it necessary to take formal action under the City's Act.
Instances were found in recently built and occupied premises where the conditions of approval
laid down by the Corporation were not fully complied with. These conditions are not obligatory
but where the opportunity presented itself occupiers were recommended voluntarily to comply
with the conditions in order to avoid the possibility of contravening the smoke legislation.
MEASUREMENT OF ATMOSPHERIC POLLUTION
The results obtained in 1957 of the measurement of rainfall and deposited matter by means
of the deposit gauge, and the determination of sulphur dioxide by the lead peroxide method are
set out in Table I below. Tables II and III record the monthly averages of the daily recordings of
the concentrations of smoke and sulphur dioxide with the volumetric apparatus installed at four
stations in the City. The measurements at the three Police Stations are undertaken by personnel
of the City Police and indebtedness is expressed to them for their continued valuable assistance.
Deposit Gauge
It is known that when a deposit gauge is exposed for a number of consecutive months on the
same site it is often found that the successive results vary considerably. This variation is due
partly to changes in the rate of emission from chimneys, but mostly to fluctuations in meteorological
factors such as rainfall, speed and direction of wind and the turbulence of the atmosphere.
Another important factor which applies, particularly in the City of London at the present time, is
the effect of the wide scale redevelopment of war damaged sites involving the demolition of old,
and the construction of new, buildings.
It will be observed that, compared with the average of the previous five years, there is an
increase of total matter deposited in the gauge. This is partly accounted for by the unusually
high proportion of insoluble matter deposited in March, namely 36.18 tons per square mile, compared
with the yearly average of 14.56 tons per square mile, despite the low rainfall in that
month. In his analysis of the water collected in March the Public Analyst commented that "the
ash of the water insoluble matter contained a considerable amount of iron rust". This was
undoubtedly due to redevelopment of war damaged sites adjacent to the deposit gauge. On the
other hand it is satisfactory to be able to report that the weight of "tarry matter" — a water
insoluble constituent which is closely connected with the emission of chimney smoke — is much
less than in preceding years. In 1957 it was 2.64 tons per square mile compared with 3.28 in
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