G34 



MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



to have been circulating very much over Switzerland. It will be seen from the table 

 i hat during the days when the circulation was mixed, and with wind on the Rigi from 

 the impure direction, the number of particles was almost always high, though it fell low 

 on two occasions, when in cloud and rain. On the 25th the wind set in from the south, 

 and for a time the numbers fell low. It was under the influence of this southerly wind 

 that Hochgerrach was seen for the only time in 1892. 



The air circulation at high and low level during the visit in 1893 was very mixed 

 and not at all unlike the conditions during the previous visit. During the 10th, 11th 

 and 12th, the air was coming from a northerly direction and the number of particles was 

 high, except on the 12th, when the hill-top was in cloud. On the 13th a southerly wind 

 began to blow, but it soon changed to an impure direction. The number of particles was 

 exceptionally high during the day, but fell towards evening. The wind returning to a 

 southerly direction, the numbers were fairly low next day, that is the 14th. The high 

 numbers on the 13th, as already explained, were due to the very mixed air circulation all 

 over Switzerland at both high and low levels. On the 15th the wind was still southerly 

 in the morning and the numbers low, but rose as the day advanced, under the influence of 

 a northerly wind. On the 16th the wind again went southerly and the dust was less 

 than on the previous day. It was only on the 13th, 14th, and morning of the 15th, when 

 the southerly wind had temporarily cleared the air, that Hochgerrach was visible. 



To show the effect of the direction of the wind on the number of particles in the 

 air on the Rigi, Tables VI. and VII. have been prepared. In Table VI. are entered 

 the observations taken after the wind had blown from a southerly direction for some 

 time, that is after the pure air had arrived at the Rigi. In Table VII. are entered the 

 observations taken when the wind was from impure directions, that is from the plains, 

 and also the observations taken when the direction of the wind was doubtful or was 



Table VI. — Shoiving the Highest and Loxvest Number of Dust Particles observed on the Rigi 



Kulm when the Wind was from the Alps. 



Date. 



Wind. 



Highest 

 Number. 



Lowest 

 Number. 



State of the 

 Air. 



22nd May 1889 

 23rd „ „ 

 24th „ „ 

 25th „ „ 

 19 th „ 1890 

 20th „ 1891 



- 1 »t ,, 

 23rd „ „ 

 24th „ „ 











S.E. 



>) 



S.S.E. 

 S.W. 



S.E. to S.W. 



w. 



2350 



1100 

 685 

 580 

 775 

 683 

 2000 

 3075 

 494 



434 

 515 

 350 

 532 

 375 

 326 

 378 

 467 

 410 



Clear. 

 Very clear. 

 )> 

 ii 

 ii 

 ii 

 ii 

 ii 

 i> 



Mean 





1305 



421 





