484 



MR AITKEN ON ATMOSPHERIC DUST. 



visibility in miles, the limit of visibility being obtained by estimating the amount 

 of haze between the observer and a mountain at a known distance. Suppose the 

 mountain to be 25 miles distant and to be half-hazed, then the limit of visibility 

 will be 50 miles. In illustration of these tables the one showing the Kingairloch 

 observations for 1893 is here reproduced. 



Table allowing the relation between the Number of Dust Particles and the Transparency of the 

 Atmosphere with Wet Bulb Depression of 7° and over. 



Date. 



Mean Number of 



Limit of Visibility 



C. 



Particles. 



in Miles. 



12 July, . 



388 



250 



97,000> 





24 June, 







508 



200 



101,600 





15 July, 







828 



200 



165,600 





3 „ 







857 



150 



128,550 





5 „ 







1480 



100 



148,000 





6 „ 







1596 



80 



127,680 



mean 

 '140,628 



11 „ 







1825 



100 



182,500 



30 June, 







3950 



60 



237,000 





8 July, 







4100 



50 



205,000 





7 „ 







5100 



17 



86,700 





1 ,, 







5175 



13 



67,275J 





When all the observations had been entered in tables similar to the above, it was 

 seen that, while the numbers in the columns of particles gradually increased, the limit of 

 visibility gradually decreased. Further, when the number of particles was multiplied 

 by the limit of visibility, a number was obtained which in a rough way was constant 

 for the different observations in each table, but this number was not the same for the 

 different wet bulb depressions. In the above table there is a column headed C. In 

 this column is entered the number got by multiplying the number of particles into the 

 limit of visibility, and it will be noticed that the value of C is far from constant ; but 

 this is only what might be expected, owing to the difficulty of estimating the density of 

 haze, and lack of knowledge regarding very much of the intervening air, which may 

 differ much from that at the place of observation. 



Though the different individual observations in the above table give far from the 

 same value for the constant C, yet when we average a number of observations, even 

 though those observations are taken at different places and the haze has to be estimated 

 under different conditions, the agreement is, considering the difficulties, fairly close. This 

 is seen in the following table, in which is arranged the mean values of C from the obser- 

 vations made at different wet bulb depressions at Kingairloch, Afford, and Rigi Kulm. 



It will be noticed that the value of C at the three stations is, considering the 

 difficulty of estimating haze, fairly constant in each column for the different humidities, 

 but that the number is not the same for the different humidities, being nearly twice 

 as great in the driest air as it is in the dampest, the explanation of this being that 

 the same number of particles has a much greater hazing effect in damp than in dry air. 



