218 
Proceedings of Royal Society of Edinburgh. [sess. I 
examination of the figures, the effects of hoth dust and humidity- 
being evident. 
The most interesting of the Alford tests are those made on the 
16th of September. On the morning of that day the air was medium 
clear, with bright sunshine, and the number of particles was 1125 
per c.c. As the day advanced the air thickened in a very remark- 
able way; from having a medium clearness in the morning it 
gradually thickened and lost its transparency as the day advanced, 
and in the afternoon it was extremely thick, but clear overhead 
and free from clouds. When the air was tested at 5.30 p.m. the 
dust particles were found to have increased to a higher number 
than had been observed at this station — the number was 5700 per 
c.c. The great increase in the thickness of the air which took place 
during the day was evidently due greatly to this increase in the 
amount of dust. Increased humidity may possibly have had some 
effect, but probably it was not the chief cause, as the wet bulb was 
only depressed 1°*5 more at 9 a.m. than at 9 p.m. 
An examination of the Dumfries series of observations bears out 
the conclusions we have come to from the other figures. The 
relation between the transparency and the dust and humidity is 
evident, but the figures obtained at the last situation are not so 
clear on this point, probably owing to the air not being so free from 
local pollution. The Dumfries observations might have been omitted 
if it were not that they give us information on some other points. 
It will be observed that the numbers at Dumfries were generally 
very high, often extremely so, for a thinly populated country 
district. An examination of the figures in the column headed wind, 
shows us that these very high numbers generally occurred when the 
wind was slight or variable, as on the 13th, 14th, 16th, 18th, and 
19th of November. The increase in the amount does not seem to have 
been entirely due to the direction of the wind bringing the air from 
impure sources, as the lower current as well as the general upper 
circulation varied on these days from south by west to nearly north. 
These large numbers may therefore have been greatly due to an 
accumulation of purely local impurities, as it will be seen that 
whenever the air was very impure it always got much purer when- 
ever the wind increased. These Dumfries observations are evidently 
of less value than the Ivingairloch ones for investigating the relation 
