MR AITKEN ON ATMOSPHERIC DUST. 



485 



Place. 



Values of C at Different Wet Bulb Depressions. 



2° to 4°. 



4° to 7°. 



7° and over. 



Kingairloch, 1893, 



1892, . 



Alford, 



Rigi Kulm, .... 

 Mean, .... 



77,525 

 No observations 

 75,474 

 75,176 

 76,058 



105,923 

 116,677 

 95,153 

 104,430 

 105,545 



140,628 

 174,832 

 124,921 

 124,211 

 141,148 



To find what is the actual number of particles required to produce a complete haze, 

 that is, render a distant mountain invisible, all that is necessary is to multiply the 

 above constant C by 160,932, that is, by the number of centimetres in a mile. When 

 that is done we get the numbers given in the following table, which shows us the 

 number of particles at the different humidities in a column of air 1 square centimetre 

 section, whatever the length of the column may be, selecting for this table the 

 observations made at Kingairloch in 1893, as they are the most to be relied on and do 

 not differ much from the average. 



Wet Bulb Depression. 



Number of Particles required to 

 Produce a Complete Haze. 



2° to 4° 

 4° to 7° 

 7° to 10° 



12,500,000 

 17,100,000 

 22,600,000 



This table shows that, roughly speaking, the transparency of the atmosphere is 

 doubled by decreasing the humidity by one-half, as it only requires half the number of 

 particles to make a complete haze with a wet bulb depression of from 2° to 4° that is 

 required when the depression is 7° to 10°. 



With the information we now have of the hazing effect of dust, it is possible roughly 

 to estimate the number of dust particles in the air without a dust-counter. As we have 

 established the numerical relation between the number of dust particles, the haze, and 

 the humidity, so that any two of these being known the third may be calculated, 

 so if we know the wet bulb depression and the limit of visibility we can find the 

 number of dust particles in the air without a dust-counter. 



A great many observations have also been made at Falkirk* on the haze in the 

 atmosphere. These observations were made without a dust-counter, with the object of 



* Proc. Roy. Soc. Edin., vol, xx. p. 76, Jan. 30, 1893, 



