ATMOSPHERE OE GREAT BRITAIN AND ON THE CONTINENT. 627 



This northerly low-level wind penetrated into the valleys to the south of the Rigi and 

 was forced up the northern face of the Alps, where, after rising to a certain height, it 

 was caught by the southerly current and carried northwards again. The sudden rise and 

 the unsteady numbers observed in the afternoon were probably due to the tests being 

 then made in impure northerly air, which was imperfectly mixed with the purer air of 

 the Alps brought by the southerly wind. 



The reasons for accepting the above as the explanation of the high afternoon readings 

 are : — First : That when the numbers went high the humidity increased greatly, the wet- 

 bulb depression of 9° falling to one of 4° when the air was impure, showing that this air 

 came from a different source from the purer air. Second ; This folding over of the impure 

 northerly air was well seen on the slopes of Pilatus. As this northerly air rose, it began 

 to condense to cloud. The lower part of this cloud moved from the north, and after it 

 ascended the mountain slopes to a considerable height it was seen to be doubled over and 

 turned back on itself, and to be moving from the south. Third : As the evening advanced 

 the northern current slackened, and from the movement of the cloud on Pilatus, and of the 

 smoke on its lower slopes, it became evident that the north wind was dying away and the 

 southerly one was again descending to lower and lower levels. The dust observations on 

 the Rigi at 7 p.m. show that the upper air was again pure, and that it had also again 

 become drier, and this change took place at the time the southerly current had succeeded 

 in beating back the northerly air. It therefore seems highly probable that the high 

 numbers observed on this day were due to an intrusion of impure northerly air into the 

 pure southerly air in the upper atmosphere. 



It may be remembered that a marked feature of the air on the Rigi during most of 

 the time of the 1890 visit was the great amount of haze then observed. This haze at 

 sunset hung like a veil between the Alps and the observer, the upper limit of the veil 

 being then distinctly visible far above the height of the highest Alp, suffused with a 

 reddish glow from the setting sun. From my notes I find that none of this coloured 

 haze was visible during the first days of the 1891 visit; the veil was removed and the 

 upper air was uncoloured by the setting sun. If we look at the figures for the dust 

 particles in 1891 and 1890 we shall find that the year this haze was so marked the 

 number of particles was very great. During the hazy days in 1890 the number never 

 fell to 1000, while during the first days in 1891 it was generally under 500. 



Returning to the 1891 observations, Table I., the 22nd was ushered in with a change 

 of weather for the worse. The wind had shifted to the west during the night, and it was 

 snowing at 9 a.m., and it continued to rain, hail or snow at intervals during the day. 

 The number of particles in the morning was small, being 300 per c.c. By mid-day the 

 impure north-west wind seems to have arrived at the Kulm and the number rose to 1400. 

 At 2 p.m. the numbers had become irregular, more irregular even than shown in the table, 

 these not being the extreme numbers observed. In the evening the wind had fallen, and 

 the impure air was no longer driven to the mountain top, the number falling to about 

 400 per c.c. 



