26 MR JOHN AITKEN ON THE NUMBERS OF DUST PARTICLES IN THE 



been thought necessary to enter them all in the table. As might have been expected 

 from the irregularity of the movements of the air, the numbers varied greatly at short 

 intervals. The highest number observed was 1275 per c.c., and the lowest 625 per c.c. 



Dust and Wind on the Rigi. 



During the first visit to the Eigi the air was generally clear, whereas during the 

 second it was almost always very thick. On looking for an explanation of the greater 

 thickness on the second occasion, we may suspect two things, either together or separately, 

 as the cause of the increased thickness, — either there was an increase in the humidity or an 

 increase in the dust, or an increase in both. The observations show no sufficient increase 

 in the humidity to account for it, while the dust observations show a vast increase in 

 the number of particles. The question now is, What was the cause of the greater 

 number of particles during the second visit ? One naturally expects that the force and 

 direction of the wind will have an important influence on the amount of dust. We 

 have previously seen that increase of wind reduces the amount of dust at low level. This 

 gives rise to an increase at high level when the wind first begins to blow, though after 

 it has blown for sometime it causes a decrease of dust at high level also. The direction 

 of the wind, however, has an important influence at this station, as all to the north of 

 it is densely inhabited, while the Alps close in round it to the south. 



An examination of the air circulation during the two periods has been made from the 

 weather charts of Switzerland kindly supplied to me by M. Billwiller. Selecting the 

 meteorological stations surrounding the Rigi, an examination was made of the force and 

 direction of the winds during the periods to see if there were less wind, or if it blew more 

 frequently, from the N., i.e., from polluted areas, in 1890 than in 1889. No satis- 

 factory explanation of the difference was obtained from this examination, partly because 

 the information about the winds, as regards their force, is too slight, but principally 

 because the winds were generally light and variable. And often the direction at one 

 station bore no relation to the direction at the others, and at two adjoining stations the 

 wind would often be blowing in exactly opposite directions. No satisfactory conclusion 

 could, therefore, be drawn from these charts worked in that way. One interesting point, 

 however, came out from an examination of the winds during these periods. It was found 

 that the wind had a decided tendency to set in towards the S. during the day, and 

 from the S. at night — i.e., when the morning observations were taken at 7 a.m. 

 the direction was frequently southerly, i.e., from the Alps; and when taken at 1 p.m. 

 it had frequently changed to northerly, i.e., to the Alps. One is quite prepared to 

 find this up and down movement near mountains, but one would scarcely expect to find 

 it takes place over a great part of Switzerland. 



From these remarks it is evident that no satisfactory explanation of the difference on 

 the two occasions could be obtained from an examination of the winds at all the stations. 

 Another plan was then tried, and the winds of only the high-level stations examined. 

 The true explanation was likely to be obtained from them, as they would show the 



