ATMOSPHERE IN GREAT BRITAIN AND ON THE CONTINENT. 27 



general circulation of the air, while the low-level stations gave frequently purely local airs. 

 The high-level stations near the Eigi area, and suitable for our purpose, are the Santis, 

 8215 feet; the St Gothard, 6935 feet; and the Rigi, 5905 feet — using for the latter 

 station my own observations, as none are entered in the Swiss Records for this station 

 on these dates, the telegraph not being in use till later in the season. 



An examination of the winds at the high-level stations at once showed the cause of 

 the difference on the two occasions. During the visit in 1889 the wind was always 

 southerly, whereas during the second visit it was frequently northerly. Entering more 

 into detail, I find that on the morning of the day I arrived, on the first visit, the wind 

 at the St Gothard was blowing strong from the St, and that it continued to blow from 

 that direction, and generally with some force, all the days of the visit. At the Santis it 

 was generally southerly, but occasionally it went a little E., or W., of S.; while on 

 the Rigi it was always E. of S. — i.e., during the first visit the upper wind always came 

 from the mountains, and brought pure air to Switzerland. On the occasion of the second 

 visit, when the observations were begun on the 15th May, the wind was northerly on 

 the St Gothard and the Rigi, and westerly on the Santis. The wind continued in much 

 the same direction on the 1 6th at the St Gothard and the Santis, while on the Rigi it 

 was southerly but light. On the 17th the wind was still northerly on the Santis and 

 Rigi, but showed a tendency to change on the St Gothard, there being a slight southerly 

 air by mid-day. During the days of northerly circulation the number of dust particles 

 was much greater than in the previous year, when the wind was southerly. During the 

 continuance of the N. wind, with its great number of particles, the air remained thickly 

 hazed and the hills veiled. The 18th brought a change in the conditions. In the 

 morning it was blowing fresh from the S. at the St Gothard, and the wind also had 

 changed to southerly on the Santis and Rigi. By mid-day there was only a slight 

 reduction in the dust on the Rigi. The impure air seemed to be still passing, but by the 

 evening the great impurity brought up by the northerly winds was rapidly being 

 cleared away, the dust particles having fallen from a maximum of 3800 to 725. It is 

 possible that part of the reduction of the dust may have been due to the heavy hail-shower 

 already referred to, but its effect would be only temporary. On the 19th, the wind 

 continued to blow from the S. at all the stations, and the number of particles continued 

 to fall, and fell quite as low as on the previous year ; the air also became as clear, the 

 distant mountains being quite as distinct as on the previous visit. It seems, therefore, 

 probable that the clearing of the air was not due to the thunderstorm which took place on 

 the 18th, but to the change of wind bringing purer air from the unpolluted area of the Alps. 

 It is interesting to note that on this occasion the thunderstorm took place where the con- 

 tending pure and impure currents met. So long as the storm was to windward, the number 

 of particles was high ; but in the immediate rear of the storm the air was pure. 



On the morning of the 20th the number of particles showed a decided tendency to 

 increase, and was very high by mid-day. The wind on the St Gothard was still 

 southerly; on the Santis it was S. W.; and on the Rigi, E. But while the upper current 



