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Since October, 1955, regular observations of the daily average
concentration of smoke and sulphur dioxide have been made by means of
apparatus installed in the Municipal Offices. The smoke is measured
by drawing a measured volume of air through a white filter paper in
such a way that the entire solid impurity from the air remains in the
filter paper as a grey stain. The weight of material caught is estimated
by matching the stain with a scale of shades, which has been previously
calibrated by matching with weighed stains. The sulphur dioxide
is measured by drawing air from outside the building at a steady rate of
about 50 cu. ft. per day through a hydrogen-peroxide bubbler, the
volume of air being measured with a gas meter. The bubbler contains
25 ml. of dilute hydrogen peroxide, and sulphur dioxide is
removed from the air to form sulphuric acid. At the end of a day's
run the volume of standard alkali needed to neutralize the sulphuric
acid is measured. The average concentration of sulphur dioxide
during the period of the test can then be calculated by formula.

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