1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
28 7 
by a fall in dust, and a fall in wind with an increase of dnst. It 
seems therefore that the amount of dust is not so much a question 
of cyclonic or anticyclonic areas, as of wind velocity. It will be 
noticed that the exceptions in the Dumfries observations on the 
25th October and 9th November,, when the numbers were small and 
the distribution of pressure anticyclonic, have no existence where 
we adopt the wind explanation. 
Having traced the connection between the number of particles 
and the velocity of the wind at low levels, the question still 
remains, What is the action of the wind ? Two explanations seem 
possible. The wind will evidently mix the upper and lower airs, 
and may thus prevent the number being great, either by preventing 
the dust in the upper atmosphere from settling into the lower, or 
it may prevent the accumulation of local impurities, from which 
no locality is entirely free. The latter of these suggestions seems 
to be the more probable, as the atmospheric dust is so excessively 
fine that it is likely to take long to settle, and the duration of calms 
does not seem long enough for it settling from the upper strata. 
There is no doubt that for all the dust to settle out of even a few 
feet a very long time is necessary. It must, however, be kept in 
mind, that while all the dust takes long to settle, the heavier 
particles will fall quicker, and these no doubt in calm weather will 
add something to the impurity of the lower air. It, however, seems 
probable that the absence of wind will act chiefly in allowing local 
impurities to accumulate and keep near the ground. It may be as 
well to recall here, that on the Rigi Kulm, the lowest number of 
dust particles was recorded in calm weather ; and that with increase 
of wind increase of dust took place, which increase continued 
till the wind had blown some time, thus indicating the power of 
the wind to make the upper air impure, and the complement of this 
will be to make the lower air purer. 
That the increase of dust during calm periods is principally due 
to the accumulation of local impurities, seems to be supported by 
the difference, already pointed out, of the effect of wind as shown 
in the three diagrams. At Dumfries the relation between the 
amount of dust and wind velocity is constant all through, while at 
Kingairloch and Alford there were marked exceptions to it. Now, 
Dumfries is a much more polluted district than the others, and it is 
