238 Proceedings of Royal Society of Edinburgh. [sess. 
polluted in all directions, whereas the other stations are not polluted 
equally in all directions. It seems, therefore, to he on this accumu- 
lated local impurity that the wind acts. 
Another point which is shown in the diagrams Nos. 1, 2, and 3, 
is the influence of the direction of the wind on the amount of dust. 
At the foot of these diagrams are entered arrows showing the direc- 
tion of the wind at the dates. The lower arrow shows the direction 
of wind as observed at the station. The upper one is from the 
weather charts issued by the Meteorological Office. These upper 
arrows will probably represent the general air circulation, while the 
lower ones will frequently be purely local. It will be observed 
that when the wind came from populous parts of the country, the 
numbers were generally large. Taking the Kingairloch observa- 
tions, — all the country from the south to the east is thickly popu- 
lated, but in all other directions there are but few habitations. On 
the 9th, with a light wind from the south, the number was great ; 
whereas, with a light wind from the east or north, the number was 
less. The high numbers were all with southerly winds. At Alford, 
the most densely populated direction is from east to nearly south, 
and it will be observed that it was with winds from these directions 
that the greatest amount of dust was observed. The low numbers 
observed at this station, when the wind was slight from the 11th to 
the 14th, was probably due to the air coming from a comparatively 
poorly populated district. This seems to explain why on these 
days the number of particles was not great, as is generally the case 
when the wind falls. It will be noticed that the wind was south 
when the great increase of dust occurred on the 16th, with increase 
of wind. The effect of the direction of wind is not so apparent in 
the Dumfries observations, probably owing to this station being 
surrounded on all sides by towns and villages. It is, however, 
interesting to note that the northerly winds which were pure at the 
other stations were found to be impure here. The reason for this 
probably is, that many towns, as well as iron and other works, lie 
in that direction. 
Fog. 
The condition of the air during fog has been tested in a number 
of cases, of which no record is given here, and in all of them there 
was found a great quantity of dust. This is what we should now 
