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1956 and 3.45 — the average of the years 1952-1956. The principal soluble constituent of the
rainwater collected by the deposit gauge is sulphur dioxide. In 1957 the monthly average was
30.16 tons per square mile compared with 34.69 tons for 1956 and 32.39 tons — the average for
the years 1952-1956.
Sulphur determination by the lead-peroxide method.
This method is designed to provide a useful index of the activity of sulphur pollution on the
fabric of buildings by ensuring that no major change in such pollution shall go undetected. The
results in Table I show that no such major change took place in 1957 when compared with the
average of the previous five years.
Volumetric determination of smoke and sulphur dioxide.
This method gives the approximate concentration of sulphur dioxide and smoke. Unlike the
deposit gauge and the lead peroxide cylinder this method of determination is not unduly influenced
by weather conditions. The deposit gauge gives no information about pollution by smoke
hence the necessity of the smoke filter to provide a means of assessing local and national smoke
problems and the rate of progress towards a cleaner air.
Sulphur.
The concentrations of sulphur dioxide, as shown by the results set out in Table III, were
less in 1957 when compared with 1956. At Golden Lane the monthly average of the daily readings
in 1957 was 9 parts per 100 million parts of air. The average for the five previous years
was 11.
Smoke.
There was a marked decrease at Golden Lane — 9 milligrams per 100 cubic metres of air
compared with the average of 13 for the previous five years. No reliable comparisons can be
made for the other three stations since they have not been established for a sufficiently long
period.

18.