1889-90.] Mr J. Aitken on Dust Particles. 231 
1 ° C. Now note the difference, the air to the south did not thicken 
in the evening, this was owing to the air on this side of the 
mountain getting dryer as the evening approached. 
On the morning of the 23rd the air low down was thick all 
round, owing to the humidity being high on both sides of the 
mountain. At Lucerne the depression was only 2° C., and at Gersau 
only 1 0, 4. In the afternoon matters changed, and became much the 
same as they were on the afternoon of the previous day. The air 
to the south was clear low down, hut to the north it was thick and 
hazy. To the north the wet bulb depression was 5° *2 C. at 1 p.m., 
and fell to 2° at 9 p.m., while in the south it was 3° *7 C. at 1 p.m., 
and increased to 7° '7 C. in the evening. Here again may he ob- 
served the same relation between the transparency and the humidity. 
The only difference of importance in the air on the 22nd and 23rd 
was a strange blackness in the air to the north on the 23rd which 
had not been observed on the previous day. 
On the 24th and 25th nothing special was noted in the lower air 
in the different directions. On both of these days the air was clear 
both high up and low down, and on looking at the Swiss Meteoro- 
logical Report I see the humidity was low on these days in both direc- 
tions. At Lucerne the wet bulb depression at 1 p.m. was as much as 
7 °*8 C. on the 24th and 8°’4 C. on the 25th, and at Gersau it was 9°T 
C. and 9° -6 C. respectively, and in the evening the air kept fairly dry 
at both places. All through these observations of the air below the 
mountain we find the relation between the humidity and the trans- 
parency to hold good. Increased dryness was always accompanied 
by increased transparency. I was much struck while making these 
observations with the difference in the appearance of the air low 
down when looking north and when looking south of the mountain, 
and thought it might be due to the amount of dust in the two 
directions, the thickest air being towards the inhabited parts of 
the country. At the time I did not expect this difference in the 
humidity of the air at the two places which the Swiss observations 
show. It would be difficult to trace the cause of this difference in 
the humidity of the two places which are so near each other. Any 
attempt to trace it to direction of wind would be difficult owing to 
the influence of the surrounding mountains in producing local 
currents. It is quite possible the difference in the air to the north 
