Mr J . Aitken on Dust Particles. 
251 
1889 - 90 . 
APPENDIX. 
Being desirous of getting more observations on the effect of 
humidity when the temperature is low, in order to check the con- 
clusion come to in the paper, I made a visit to the west of Scotland 
in the end of January last, in the hope of finding the conditions 
suitable for the purpose. Garelochead was selected for these obser- 
vations. This situation is fairly free from local pollution so long 
as the wind is not south, south-east, or east. In all other directions 
it is fairly free from contamination. 
The observations were generally made on the hills above Gare- 
lochead ; the site for each day being always selected to the wind- 
ward of the houses. Unfortunately, the weather was not suitable 
for the special purpose intended, as during most of the time the 
temperature was high and the weather extremely stormy. As, 
however, the observations were taken in exceptional weather, and 
the results show peculiarities, they may be thought of sufficient 
interest to be recorded here. 
The observations were begun on the 23rd of the month. On this 
day the number of particles was 2360 per c.c., and the temperature 
34° ; but unfortunately the wet-bulb depression is doubtful, owing to 
the temperature of the wet-bulb being below the freezing point. The 
wet-bulb fell to 32°, where it remained steady. As the temperature 
was falling fast, the conditions would be all changed before the 
water on the wet-bulb was all frozen, and it was therefore useless 
to wait to see how far it would fall below 32°. Fortunately, I 
had a hygroscope exposed at the same time, and it showed a reading 
corresponding to a wet-bulb depression of fully 3°. 
This is the only observation obtained at this situation suitable 
for illustrating the influence of temperature on the effect of the 
I humidity, and it confirms the conclusion previously arrived at. 
We have shown that when there was a wet-bulb depression of 
about 4°, and a little over 1000 particles per c.c., that the air was 
thick if the temperature was 60° or more ; clear or medium if it 
was about 50°; and, in the case here recorded, the air was clear, with 
nearly double the number of particles, and the air not quite so dry, 
but at a temperature of 34° It seems probable from this that 
observations made on Ben Nevis and in cold climates generally 
