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XX. — On the Particles in Fogs and Clouds. By John Aitken, Esq., F.R.S. 



(Read 6th February 1893.) 



Cloud Particles. 



In May 1891 I communicated a paper to this Society "On a Method of Observing 

 and Counting the Number of Water Particles in a Fog," and in July of the same year 

 a paper " On the Solid and Liquid Particles in Clouds." One conclusion to which the 

 observations contained in these papers pointed was, that there existed a relation between 

 the thickness or density in clouds and fogs and the number of water particles present. 

 Though the figures did not show that this relation was very close, yet in all the fogs and 

 clouds tested there was a rough relation between the thickness of the air and the number 

 of water particles observed. 



In May 1892 I again had an opportunity of making observations on cloud particles 

 on the Rigi Kulm. The morning of the 21st of that month opened cloudy, and the top 

 of the mountain was in cloud. There was slight rain, and a strong wind from W.N.W. 

 The observations on the water particles in the clouds drifting over the hill-top were begun 

 at 8 a.m., but counting was very difficult, owing to the drifting action of the wind 

 making the rate of fall very irregular. The result of the most reliable of the observations 

 was, that when the limit of visibility was 50 yards, the water particles fell at the rate of 

 from 60 to 100 per square millimetre per minute, the number, however, varied more 

 rapidly than usual ; and when the limit of visibility extended, the number of particles 

 falling was less. Though these observations were very unsatisfactory, owing to the 

 high wind blowing at the time, yet they gave a result not very different from that 

 previously obtained. 



By 10 a.m. the lower limit of the clouds had evidently risen to near the level of the 

 top of the mountain, as occasional glimpses were obtained of the valleys and lakes ; but 

 when the hill-top was in cloud the air was fairly thick, the limit of visibility being 

 about 100 yards. When the observations on the cloud particles were begun under the 

 changed conditions, I was astonished to find that I could not see a single water particle 

 falling on the micrometer, though from the thickness of the air I had expected to see not 

 a few. The number of dust particles was very high and very variable at the time, yet 

 the thickness of the air seemed far too great to be due to dust alone. The observations 

 on the water particles were therefore continued, but they were now made with the 

 utmost care. The micrometer was cooled with snow, and the focussing of the lens was 

 most carefully done. When these precautions were taken, a new condition of matters 



VOL. XXXVII. PART II. (NO. 20.) 3 Q 



