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III. — 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. Plate.) 



Part II. 



(Read 4th January 1892. ) 



In a new investigation of this kind it is always desirable to repeat the observations 

 under as many conditions as possible. The variables are so many that with a limited 

 experience it cannot be expected that the subject will be exhausted, or that the 

 conclusions arrived at from early observations will be in all cases confirmed. As an oppor- 

 tunity offered in the beginning of 1890 for repeating the tests made the previous year on 

 the amount of atmospheric dust at different places on the Continent, it seemed desirable 

 that the old ground should be gone over again rather than that the investigation should 

 be extended to new areas. The observations made in this country have also been 

 confined to the same stations as in 1889 ; and in this paper I intend giving the results 

 of a series of tests repeated at the same stations, at about the same dates, but under 

 the conditions existing in 1890, as has already been given for 1889 in Part I. of 

 this subject. 



At the end of this paper is given a table in which are entered the places where 

 observations have been made, the date and hour when the observations were taken, the 

 direction and force of the wind, the temperature and humidity of the air, and the trans- 

 parency of the atmosphere at the time. It has not been thought necessary to occupy 

 space by entering in the table all the observations made at the different places ; only a few 

 of them taken at some of the stations are given ; the others being similar and having no 

 special interest are omitted. At Hyeres, tests were made from the 26th March to the 3d 

 of April 1890. The general result was somewhat similar to that given in the previous 

 paper. The highest number observed on Fenouillet was 15,000 per c.c, the wet-bulb 

 depression 5°, the result being a very thick haze. The lowest number observed was 725 

 per c.c, with a wet-bulb depression of 9*5°. On this occasion the air was very clear, the 

 wind being from the S.W. and strong. The other observations made at Hyeres have no 

 special interest, and are not entered in the table. At Cannes, observations were made 

 on only two days, and the results call for no special remark. 



Observations were made at Mentone from the 11th to the 19th of April. The tests 

 were made on a hill about 800 feet high to the N.W. of the town. The number of 

 particles varied greatly with the direction of the wind, being as high as 26,000 

 when the wind was S.E., i.e., from the direction of Mentone, while the number fell 

 to a little over 800 when the wind was northerly, or from the mountains. This does 

 not come out in the table, as the directions of the winds entered in the table are the 

 directions from which it was blowing at the place of observation ; but as it was situated 



VOL. XXXVII. PART I. (NO. 3). E 



