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62
visibility and in the actinic power of the light. The average area of visibility in
London at mid-day is no less than 50 times greater in the three summer months,
June to August, than it is during the three winter months, December to February,
and about five times greater than it is during the three months of spring and
autumn respectively.
The actinic power of the light is more than three times greater in the summer
than in the winter months, and about twice as powerful in summer as in the spring
and autumn months. The deposit from dust and tarry matter by filtration of the
air through white filter paper during the winter months gives an average tint nearly
three times darker than the corresponding average for the summer months, while
the average of the spring and autumn tints is nearly twice as dark as the average
of those of the summer. While these seasonal changes in visibility and actinicity
are due largely to natural conditions lying wholly outside of human control, there
can be no question that coal smoke which is the main factor in the varying seasonal
changes in the filter tint is an important and aggravating factor in the screening
of actinic light and in intensifying the obscuration arising from the seasonal atmospheric
changes to which London is subject by reason of its situation in the valley
of a large river.
This is strikingly borne out by a consideration of the changes in shape of the
area of visibility. It is traceable in the composite diagrams, which give the averages
of the monthly or quarterly observations when considered in conjunction with the
averages of the direction of the wind, and is distinctly to be observed in the daily
charts in which wind direction is simultaneously recorded. The bulges of extended
visibility in one direction or another are generally towards the quarter from which
the wind is blowing. In a great city like London it is obvious that the smoke
screen will be densest in the direction of its drift, and that visibility should be
limited in such a region is only what might be expected.