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XIII. — On the Solid and Liquid Particles in Clouds. By John Aitken, Esq. 



(Read 6th July 1891). 



Towards the end of May of this year I made my third visit to the Eigi Kulm, for 

 the purpose of continuing my observations on the amount of dust in the atmosphere and 

 other meteorological phenomena. It was with a feeling of no little satisfaction that I 

 found myself at that elevated situation during broken weather, and under conditions 

 very different from any previously experienced by me. On this occasion I had come 

 in the hope of finding opportunities for making some observations on the conditions 

 existing in clouds, in addition to the usual dust observations, and had brought with me 

 the small instrument for observing the water particles in a fog, described in a previous 

 communication to this Society. As the weather continued to be variable during the 

 week of my visit, I was fortunate enough to succeed in making a number of interesting 

 observations on the water particles in clouds, and also of comparing the conditions 

 in clouds at this elevated situation with those previously observed in fogs at a low level. 



Before giving the results of my observations on the water particles, it may be 

 desirable to make a few remarks on the solid or dust particles. When making the 

 ordinary dust observations this year, it was frequently noticed that, when surrounded 

 by clouds, the number of particles varied greatly at short intervals of time. Even when 

 making, in quick succession, the ten tests from which the average was obtained in the 

 usual way, it was sometimes observed that the ten numbers varied more than usual. I 

 had previously found when working at elevated situations, that the numbers were fairly 

 constant for intervals of an hour, and often for many hours, whereas in clouds they 

 were observed to vary every few minutes. 



There are two ways of investigating the cause of this variability in the number of dust 

 particles in the different parts of the same cloud. By the first we may proceed by 

 observing the number of particles, and noting the condition of the clouds at the same 

 time as regards density, &c. This plan, however, requires two observers, one counting 

 the particles, the other observing the other conditions. The other and simpler plan is to 

 select extreme conditions, and to observe the air in a cloud and the clear air immediately 

 outside of it. For my first observations of this kind I had to descend the hill some 

 distance to get the required conditions, the top being covered with a continuous mass of 

 cloud. Near the lower limit of the cloud the difference in the amount of dust in the 

 clear air underneath and in the cloud was quite marked. There were about twice as 

 many particles in the cloud as in the clear air. On this occasion the clouds were 

 clearing away, and there would therefore previously have been a good deal of mixing of 

 the cloudy with the clear air ; no very great difference could therefore be expected, 

 though there was quite enough to encourage further investigation. 



VOL. XXXVI. PART II. (NO. 13.) 3 C 



