658 



M15 JOHN A1TKEN ON THE NUMBER OF DUST PARTICLES IN THE 



July, and from the 6th to the 8th July, two well-marked periods of impure air, as shown 

 by the height of the dust curves in Diagram III. 



Alford. 



The last set of observations entered in Tables I., II. and III. are those made at 

 Alford, in Aberdeenshire. They were all made in the month of September of the different 

 years. It will be seen from these tables that the air at this station is generally very 

 pure, unless when the wind is southerly. With all but southerly winds the number of 

 particles is seldom over 500 per c.c, but when the wind goes to that direction and blows 

 from inhabited areas the number generally rises to some thousands. The relation be- 

 tween the transparency of the air and the number of dust particles has also been worked 

 out from these Alford observations, and the results are given in Tables XV., XVI. and 

 XVII. The observations in these tables are arranged in the same manner as already 

 explained when treating of the Kingairloch tables. 



The Alford observations are not so trustworthy as those made at Kingairloch, very 

 few estimates of haze having been made in miles, owing to there being no suitable 

 mountain visible from the former station, and the transparency of the air has generally 

 been entered in the tables simply as Thick, Medium, Clear, &c. ; and in putting these 

 into miles for Tables XV., XVI. and XVII. I have used as near as possible the equivalent 

 values adopted in the other sets of observations.. The occasions when the air was very 

 clear are entered as 250 miles limit of visibility, clear as 100 miles, and so on. All the 

 suitable observations in Tables I., II. and III. taken at Alford in the different years have 

 been treated as one set, and not separated into tables for the different years as was done 

 for the Kingairloch observations. 



It will be noticed that the values of the constant C for the different wet-bulb depres- 

 sions are not quite the same as those given in the Kingairloch tables for 1893. When 

 the wet-bulb depression was 4° and under, values in the two tables agree fairly well, the 



Table XV. — Shoiving the Relation between the Number of Dust Particles and the Transparency 

 of the Atmosphere at Alford ivhen the Wet-Bulb Depression tvasfrom 2° to 4°. 



Date. 



Lowest 

 Number. 



Highest 

 Number. 



Mean 

 Number. 



Limit of 



Visibility in 



Miles. 



C. 



2nd September 1893 



231 



245 



238 



250 



59,500 1 





1st 



248 



252 



250 



250 



62,500 





7th 



490 



554 



522 



200 



104,400 



Mean, 



^ 75,474 



L'Tth „ 1892 



580 





580 



100? 



58,000 



8th „ 1893 



675 



800 



737 



100 



73,700 



23rd „ 1892 



1150 



1G75 



1412 



60 



84,720 





23rd „ 1891 



2850 





2850 



30 



85,500 





