626 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



valley, as it will be seen that the number of particles only fell at 2000 feet to 64 per 

 cent, of what it was at low level, and that when the air did not come up the slopes the 

 upper air was much purer, the number of particles at 2000 feet falling to 30 per cent, 

 of what it was at low level. The tables are not complete enough for working out satis- 

 factory averages, and the figures are only given as rough approximations. 



These observations show a decided tendency for the number of dust particles to 

 decrease as we ascend into the higher atmosphere. They at the same time show that 

 the decrease takes place very irregularly under different conditions. The air in all these 

 tests increased but little in purity when the lower air was, owing to the direction of 

 the wind, forced up the mountain slopes. In these cases the impure air, which was near 

 the ground at low level, got more or less mixed with the purer air over it as it rose up the 

 mountain slopes, and in this manner the amount of dust was only reduced at 2000 feet 

 to 0"64 of the original amount. If, however, the air tested at the higher elevations did 

 not come from low level, but was true upper air, the number at 2000 feet was only 0'3 

 of what it was at low level. It is necessary that we keep in mind that this decrease in 

 dust with elevation refers to an area in which considerable pollution is taking place at 

 low level, and does not apply to such places as Kingairloch, where the local pollution is 

 very slight. 



Rigi Kulm, 1891. 



All the five visits to the Rigi Kulm in the different years were made in the month of 

 May, in the middle or latter part of the month. In 1891 I arrived there about mid-day 

 of the 19th, having previously stopped a short time at Vitznau to test the air at low level. 

 At Vitznau the air was found to be fairly pure ; there were only 2100 particles per c.c. 

 The air, however, was thick, owing to it being very damp. On arriving at the top of the 

 mountain, the number was found to be 417 per c.c. During this afternoon it snowed a 

 good deal, but cleared up in the evening. The air continued pure all the afternoon, and 

 after the snow ceased it was very clear, the most distant Alps being quite distinct. 



The sunrise next morning, the 20th, was very fine, the mountains being free from clouds 

 and the air remarkably clear. These conditions remained all day. The number of 

 particles during the time was small, being 683 in the morning and fell to 326 in the 

 evening, the fall being due to an increase in the pure S.W. wind then blowing. 



The 21st was also a remarkably fine day with clear air and but few clouds, though 

 they were beginning to show a tendency to form. The wind continued to blow from the 

 southwards, and during most of the day the number of particles remained low, varying 

 from 400 to 800 ; but in the afternoon, from about 4 p.m. to 6 p.m., the wind showed a 

 tendency to become very irregular, and the number of particles to increase. This 

 -uggostcd that the rise in the dust might be due to local pollution, and the site of obser- 

 ve I ion was changed, across wind, to different points, but no pure air or steady numbers 

 were observed. On looking down, however, to the lakes and valleys, it was seen that a 

 strong northerly wind was blowing at low level, while on the Rigi it was southerly. 



