ATMOSPHERE OF GREAT BRITAIN AND ON THE CONTINENT. 637 



Daily Variation of Dust on the Rigi. 



The 1889 observations show very little evidence of a daily maximum of dust. This 

 is owing to the strong southerly wind which continued to blow during all the days of 

 the visit in that year. The southerly wind kept the air in the valleys pure, and if the 

 strong wind permitted any air to ascend, it did not bring much dust with it to the Kulm, 

 The 1890 observations, however, show that on all the days, except the 19th, when a strong 

 southerly wind was blowing, the number of particles was much greater during the 

 day than in the morning, being from two to four times greater. During the visit in 1891 

 the daily maximum was not well marked on the first three days, that is the 19th, 20th and 

 21st. Under the influence of the strong winds from the pure area the day maximum 

 appears only for a short time on one of the days. The 22nd being cloudy and rainy, it is 

 again only slightly marked. On the 23rd the daily maximum was well marked by mid- 

 day, but a strong southerly wind beginning to blow, it was checked, and the number 

 again fell. On the 24th, under the influence of cloud and rain, the daily maximum is 

 entirely checked ; but the 25th being fine, with but little wind, the daily maximum 

 again asserts itself, till checked by cloud at mid-day. The daily maximum in 1891 is 

 thus only slightly indicated owing to unfavourable conditions for the lower air rising, 

 namely, cloudy skies and strong winds. 



In 1892 the daily maximum is very well marked on the 18th, 20th, 21st, 22nd and 

 24th. On the 19th the rise is only slight, owing to clouds. On the 23rd it does not show 

 at all, owing to the day being cloudy and wet. On the 25th, owing to a southerly wind, it 

 is only slightly marked. 



Coming now to the 1893 observations, it will be seen that the daily maximum was 

 well marked on all the days of this visit except on the last, when the observations were 

 stopped at 1 p.m. On that day the number had risen but slightly. 



The amount to which the daily maximum increases varies greatly ; frequently it is 

 only three or four times the morning number, but it has been observed as high as eight 

 times the morning number, as, for instance, on 21st May 1892 ; the morning number on 

 that day was 925, maximum at 9.40 a.m. 7700, and 4 p.m. 7650. On 12th May 1893 

 the morning number was 690, and the day maximum at 3 p.m. 5700. 



The hour at which the number begins to increase is very irregular — sometimes a rise is 

 evident by the time of the second morning observation, that is by 9 a.m., and sometimes it 

 is the afternoon before the impure air comes to the top of the mountain ; but the maximum, 

 if not checked by clouds, is generally attained sometime in the afternoon. As a consecmence 

 of this, the evening numbers are seldom as low as the morning ones. This irregularity in 

 the time of the appearance of the impure air at high level might have been expected, as 

 its ascent is governed by so many variables, such as amount of sunshine on the slopes 

 of the mountain, the hour at which it begins to heat the lower air, and the force and 

 direction of the wind. 



VOL. XXXVII. PART III. (NO. 28). 5 D 



