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XXVIII. — On the Number of Dust Particles in the Atmosphere of certain Places in 

 Great Britain and on the Continent, with Remarks on the Relation between 

 the Amount of Dust and Meteorological Phenomena. By John Aitken, F.R.S. 

 (With Plates.) 



Part III. 



(Read 19th February 1894.) 



In the two papers previously communicated to this Society under the above title, 

 Parts L* and II., t I have given the results of my observations on the dust in the atmo- 

 sphere for the years 1889 and 1890. In this paper will be given the results of similar 

 observations I have been able to make during the years 1891, 1892 and 1893. The 

 observations made during these last three years are similar to those made in the two 

 previous ones ; and were mostly made at the same places and at about the same dates as 

 those already given. These five sets of observations are therefore comparable with each 

 other, and give a fair statement of the number of dust particles in the atmosphere at the 

 different places at the particular dates. 



All the dust observations have been made with the same instruments as were used 

 in the previous observations. Most of the tests were made with the Pocket Dust 

 Counter, as it is the most easily worked, but the portable form of the instrument has 

 also been occasionally used ; and at times both instruments were used, to check any 

 defect there might have been in the working of either. During the making of many 

 hundreds of observations no disagreement has been found between the figures given by 

 the two instruments, other than the small percentage-differences due to instrumental 

 errors of observation and the ever-changing conditions they have to deal with. This, 

 however, is only what might have been expected, as both instruments indicate when 

 they are not in working order. If all the movements necessary for a test are correctly 

 made, with neither of the instruments can an observation be taken unless it is in working 

 order. The principal enemy to be contended with in correct observing is leakage of air 

 into the receiver, as all air leakage admits dust which would invalidate the readings. 

 If, however, all the movements are correctly gone through, the presence of these dust 

 particles makes it impossible to get the air in the receiver perfectly free from conden- 

 sation when expanded, the necessary condition for it being in before beginning to make 

 a test. 



As in the previous papers, along with this one are given tables for the different years, 

 in which are entered the different observations (see Tables I., II., and III., at the end 

 of this paper). In these tables, as before, are given the place, date and hour of the 



* Proc. Roy. Soc, Edin., vol. xvii., 1890. t Trans. Roy. Soc, Edin., vol. xxxvii. part i. (No. 3). 



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



