654 



ME JOHN AJTKEN ON THE NUMBER OF DUST PARTICLES IN THE 



Humidity and Transparency. 



If we examine the numbers for the mean values of C in the tables for 1893 it will be 

 seen that it has very different values at the different humidities. When the wet-bulb 

 depression is from 2° to 4° its value is not much more than half of what it is when 

 the wet-bulb depression is 7° and over, which being interpreted is that a much smaller 

 number of particles limits the visibility in damp than in dry air. To find what the actual 

 number of particles is that produces complete haze at the different humidities we have 

 only to multiply the value of C at the different humidities by 160,932, the number of 

 centimetres in a mile. When this is done we get the following numbers of particles 

 required to produce a complete haze at the different wet-bulb depressions, and it is 

 independent of the limit of visibility. 



Table XIV. 



Wet-Bulb Depression. 



Number of Particles of Dust to 

 produce Complete Haze. 



2° to 4° 

 4° to 7° 

 7° to 10° 



12,500,000,000 

 17,100,000,000 

 22,600,000,000 



The figures given in Tables IX. to XIII. bring out, in a much more satisfactory manner 

 than anything I have been able previously to give, the relation between the haze and the 

 amount of dust, and the humidity. The effect of the dust is very evident in all these 

 tables ; the transparency decreases in all of them fairly regularly with the increase in dust. 

 Take, for instance, Table XL : with about 400 particles per c.c. the limit of visibility is 

 250 miles ; with increasing dust the limit gradually diminishes as we descend the column, 

 and at the foot of the table we find that with a little over 5000 particles per c.c. the 

 limit is about 15 miles. 



To see the effect of humidity we have only to compare Tables IX. , X. and XL It will 

 be seen from them that as the air increases in dryness the limit of visibility for a given 

 number of particles increases. The effect of the humidity can also be seen in the different 

 values of C in the different tables, and its effect is perhaps better seen in Table XIV. 

 It may be as well to note here that it is not the absolute amount of vapour in the 

 atmosphere which produces this result, — it is the nearness of the vapour to its dew point. 

 As has been pointed out in previous papers, the water vapour does not seem to act as a 

 hazing agent itself; at least we have no evidence of it acting in that way ; but it increases 

 the hazing effect of the dust by more or less of it being deposited on the particles, so 

 increasing their size and their hazing effect. If we have a high temperature, and there 



