:n4 



Mil JOHN A1TKEN ON THE 



A much more favourable opportunity of testing the relative proportions of dust 

 particles present in clouds and in clear air occurred on the 25th of the month. The sun- 

 rise on that morning had been cloudless, and the air clear ; but as the morning advanced 

 the air gradually thickened, and clouds began to form in different directions and at 

 different elevations. These newly formed clouds occasionally passed over the mountain 

 top. It was therefore unnecessary to change the site of making the observations. All 

 that was necessary was to observe the amount of dust while the cloud surrounded the 

 observer, and again when the cloud had passed. Readings were taken in this way, from 

 time to time, between the hours of 9 a.m. and 10.30 a.m.; the numbers so obtained are 

 entered in the following table, each of them being the average generally of ten tests. In 

 the table is also entered the state of the air at the time. Many more tests were made, 

 but as they were taken in more or less clouded air, and the number of particles was 

 intermediate between the extremes given, it has not been thought necessary to enter them 

 in the table. After 10.30 a.m. the clouds closed quite in, and the mountain top was in 

 dense cloud for the rest of the day. 



Table showing the Number of Dust Particles in Cloud and in Clear Air on the Rigi on May 25th. 



Hour. 



Number of Particles 

 per c.c. 



State of the Air. 



9 



1225 



Haze 





1625 



In cloud 





1025 



Clearing 





2450 



In cloud 





3250 



In dense cloud 





3450 



>) » 





1250 



Clearing 





700 



Clear 





1850 



In cloud 



10.30 



4200 



>> 



The above figures show that on this occasion there was a vast difference in the number 

 of dust particles in the clouds and in the clear air surrounding them. In this case, as in 

 all clouds hitherto tested, a greater number of dust particles was observed in the cloud 

 than in the clear air surrounding it. It may be as well, however, to note here that these 

 observations were all made in cumulus clouds, and the remark applies only to that form 

 of cloud. It seems probable that other conditions may exist in stratus and other clouds. 



When the above observations began at 9 a.m., it seems very probable that the valley 

 air had already begun to ascend, as the upper air had thickened a good deal since early 

 morning, and though no clouds had yet formed, the upper air was rapidly approaching 

 saturation. Another reason for supposing that the lower air had already begun to ascend, 

 is that the number of particles on the previous evening was only about 500 per c.c, and 

 had been about that number for a considerable time. Further, it will be observed 

 from the figures in the table, that a mass of clear air outside the cloud had only 



