(>2S MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



The morning of the 23rd was very fine, cloudless and remarkably clear, but by 7 a.m. 

 the sky was overcast and snow falling. At 9 a.m. it began to clear up. The mid-day 

 observations showed a distinct increase in haze in different directions, and the dust 

 particles, which were low in the morning, rose to 3075, the wind at the time being 

 northerly at low level. In the afternoon the air again cleared, and the numbers fell to 

 about 500. It will be seen from the table that, when the air became purer in the after- 

 noon, it also became drier, showing, as already pointed out for the observations on the 

 21st, that the pure and impure airs came from different sources. 



The sun rose in cloud on the 24th, and by 8 a.m. it was snowing, the hill-top at the 

 time being in cloud. By mid-day the snow gave place to a drizzling rain. During the 

 whole of the day the number of particles remained low, but the air was never clear, 

 owing to the place of observation being in cloud. In the afternoon of this day, while 

 the clouds were coming and going over the hill-top, it was observed that the number of 

 particles was very variable, and it was noticed that the change in the numbers took 

 place at the same time as the change in the condition of the atmosphere. On separating 

 the observations made in clouds from those made in the clear air between them, it 

 became evident that the clouded air was the more impure. The observations showed that 

 there were generally from two to three times more particles in the clouded air than in the 

 air surrounding; them. This indicated that the clouded air had come from low level. 



During the night of the 24th the weather continued to improve, and the sun rose on 

 the morning of the 25th in a cloudless sky and the air was very clear. But by 7 a.m. the 

 air began to thicken, the air on this occasion having none of the usual bluish tinge, but 

 was quite white, due probably to the particles being large owing to the high humidity at 

 the time. By 9 a.m. occasional clouds began to pass over the mountain, and, as under 

 similar conditions on the previous day, the number of particles again became variable. A 

 great many observations were therefore made on the number of particles in the clouds 

 and in the clear air outside them. The result, as will be seen from Table I., was the same 

 as on the previous day. High numbers were observed in the clouds, and lower numbers 

 in the air surrounding them. It is unnecessary further to refer to these observations 

 here, as they have been discussed in a paper* given to this Society in 1891. 



At the time of the mid-day observations on the 25th the hill-top was still in cloud, 

 but the air was much purer. It, however, seemed to be still of a somewhat mixed 

 constitution, as the number of particles varied considerably. These were the last 

 observations made on the Rigi in 1891. On descending to low level, the air in a down 

 current near Vitznau was found to have nearly the same amount of dust as at high level, 

 the average figures being at high level 597, while at low level they were 583. It may be 

 remembered that a similar result was observed when making observations, under similar 

 conditions, at this place in 1889. The air at low level was afterwards tested in a current 

 coming off the lake. In this air the number rose to 1300, and when going down the 

 lake towards Lucerne the number increased to 2860 per c.c. 



* Trans. Roy. Soc, Edin., vol. xxxvi. part ii. (No. 13). 



