London's Pulse: Medical Officer of Health reports 1848-1972
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Hints from the Health Department. Leaflet from the archive of the Society of Medical Officers of Health. Credit: Wellcome Collection, London
London County Council 1926
[Report of the Medical Officer of Health for London County Council]
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58
This close correspondence between visibility, actinicity and atmospheric impurity
as measured by filtering definite volumes of air through standard filtering paper is
illustrated by a comparison of the mean monthly figures for the three years—see
Tables I, II and III, and graphs on pages 59-61.
Table I.
Showing mean daily radius of visibility during the respective months of the period September, 1923, to August, 1926. The observations were made at midday from the Flêche of the County Hall.
| Spring. | Summer. | Autumn. | Winter. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Starch. | April. | May. | June. | July. | August. | Sept. | Oct. | Nov. | Dec. | Jan. | Feb. | |
| 1923–4 | — | — | — | — | — | - | 5.7 | 2.7 | 0.9 | 0.6 | 0.7 | 0.8 |
| 1924-5 | 1.2 | 1.8 | 6.2 | 5.3 | 7.1 | 7.0 | 4.8 | 1.6 | 1.0 | 0.8 | 0.8 | 1.1 |
| 1925-6 | 1.2 | 1.4 | 3.5 | 4.6 | 4.1 | 3.8 | 4.5 | 1.5 | 1.0 | 0.8 | 0.8 | 1.3 |
| 1926 | 1.7 | 1.9 | 3.8 | 6.2 | 5.5 | 6.9 | — | — | - | - | - | — |
Table II. Showing mean of the time, in seconds, required to produce a standard tint, using Watkins “Bee” meter. The means are based on daily observations made at mid-day with a north exposure, and are those of the respective months during the period September, 1923 to August, 1926.
| Spring. | Summer. | Autumn. | Winter. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| March. | April | May. | June. | July. | August. | Sept. | Oct. | Nov. | Dec. | Jan. | Feb. | |
| 1923.4 | — | - | — | - | - | - | 18 | 23 | 45 | 45 | 52 | 54 |
| 1924.5 | 43 | 24 | 14 | 8 | 10 | 12 | 15 | 50 | 67 | 80 | 84 | 44 |
| 1925.6 | 55 | 28 | 15 | 17 | 18 | 21 | 35 | 42 | 51 | 79 | 69 | 63 |
| 1926 | 60 | 43 | 36 | 27 | 23 | 15 | - | - | - | - | - | - |
Table III. Showing the mean of the daily deposit as recorded by Dr. Owen’s Automatic Filter Recorder during the respective months of the period September, 1923, to August 1926.
| Spring. | Summer. | Autumn. | Winter. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| March. | April. | May. | June. | July. | August. | Sept | Oct. | Nov. | Dec. | Jan. | Feb.' | |
| 1923-4 | - | - | - | - | - | - | 1.1 | 1.3 | 1.2 | 1.2 | 2.3 | 1.7 |
| 1924-5 | 2.0 | 1.4 | 1.0 | 0.8 | 0.7 | 0.6 | - | 1.2 | 1.6 | 2.1 | 1.7 | 1.0 |
| 1925-6 | 0.9 | 0.8 | 0.7 | 0.6 | 0.6 | 0.6 | 0.5 | 0.9 | 1.1 | 1.3 | 0.9 | 0.9 |
| 1926 | 0.7 | 0.6 | 0.6 | 0.5 | 0.6 | 0.6 | - | - | - | - | - | - |
A distant object is seen and distinguished from Surrounding objects only if the
light which reaches the observer's eye from it is sufficiently different in intensity or
colour. If the illumination of the object is very poor it will be indistinguishable
from its surroundings. If the object is surrounded by or seen against a light haze
it is more likely to be seen if dark or in silhouette. If between the observer and the
object there is a haze, dark or light, this will by mere obscuration and scattering
of the rays reduce the visibility of the object. Atmospheric pollution may, therefore,
decrease visibility either by decreasing the illumination due to the sun’s rays or by
interposing a more or less turbid layer of air between the object and the observer.
The most striking feature of the observations is the great seasonal range in