650 



Mi; JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



that the highest limit of visibility is always associated with the lowest number of dust 

 ] "articles. 



In Table IX. the decrease in limit with increase in dust is regular and fairly propor- 

 tional to the increase. In Table X. the decrease in limit is not quite so regular. On the 

 18th July the limit was rather low, and on the 25th it was considerably too high. The 

 reason for the limit being low on the 18th is that the air was then thickening before 

 rain, which began at 1 p.m., shortly after the observations were taken. The reason for 

 the undue clearness for the amount of dust on the 25th is that the 25th was one of the 

 abnormal days with high afternoon readings, and when the evening observation was 

 taken the number had not fallen to its lowest, as was evidenced by the fact that the 

 lowest reading on this evening was four times greater than the number next morning, 

 and there was no change in the conditions to cause the fall. It is therefore in the 

 highest degree probable that the mean number of particles in the table on the 25th is 

 too high, and that this observation should have come in higher up the table. This 

 would have made the numbers representing the limit of visibility decrease more regu- 

 larly from the top to the bottom of the column. Passing on to the observations made 

 on the 4th July, Table X., it is evident that the limit is much too low — it ought to have 

 been at least twice what it was for the number of particles. The increase in dust from 

 the previous observation in the table is not great, while the decrease in the limit of 

 visibility is very great, and I am sorry to say I cannot offer any satisfactory explanation 

 of this exceptional observation. The dust was rapidly increasing at high level, but the 

 number was not great. The only suggestion I can offer is that while the weather charts 

 show the general circulation over our area to have been N.E., the wind at low level 

 was E., and on the Ben S.E., and it seems possible that the impure S.E. wind may 

 have been blowing at a higher level than Kingairloch, but the wind being slight it was 



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

 of the Atmosphere at Kingairloch when the Wet-Bulb Depression %vas 7° and above. 



Date. 



Lowest 



Number. 



Highest 

 Number. 



Mean 

 Number. 



Limit of 



Visibility in 



Miles. 



C. 



1893. 



12th July 



24th June 



15th July 



3rd „ 



5th „ 



Gth „ 



11th „ 



30th June 



8th July 



7th „ 



1st „ 



247 



07 



245 



564 



511 



900 



L350 



:1200 



:!200 



:!700 



.•51.10 



530 

 950 

 1112 

 1150 

 2450 

 2292 

 2300 

 1700 

 5000 

 G500 

 6900 



388 



508 



828 



857 



1480 



1596 



1825 



3950 



4100 



5100 



5175 



250 



200 



200 



150 



100 



80 



100 



60 



50 



17 



13 



97,000' 

 101,600 

 165,600 

 128,550 

 148,000 

 127,680 

 182,500 

 237,000 

 205,000 

 86,700 

 67,275, 



i Mean, 

 * 140,628 



