1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
217 
This dependence of transparency on humidity may be illustrated 
by these observations in another way. Taking days on which the 
number of particles was nearly the same, but on which the humidity 
was different. On the 5th, with 1500 particles and a wet bulb 
depression of 6°, the air was thick ; with about the same number 
of particles on the 8th, but with a depression of 9°, the air had a 
medium clearness ; on the morning of the 10th, with 814 and a 
depression of only 4°, the air was medium clear ; whereas at 1.30 p.m. 
on the 16th it was very clear, with the same amount of dust, but a 
depression of 7°. With 500 particles and a depression of 4°, the 
air was clear on the 15th, but very clear on the 17th with the same 
number of particles but 8° depression. It will be noticed that 
when there was a depression of only 4° the air was thick, unless 
there was less than 1000 particles per c.c., and that if the numbers 
increased and the air was at all clear there was also an increase in 
the dryness. 
It will be observed that the number obtained on the morning of 
the 9 th does not agree very well with the others. The number on 
this morning was high and the dryness not very great, and yet the 
air had a medium clearness. We, however, leave the figures in the 
table, and the only explanation we can offer is that this number 
was local and due to an almost entire absence of wind at the time, 
though it must be admitted that the numbers are rather constant to 
be explained in this way. 
The Alford observations point to the same conclusions. I must, 
however, state here that the figures entered in the columns headed 
temperature and humidity were not obtained by myself at the 
place and hour of observation, but are from the weather report 
of the Scottish Meterological Society’s Station at Logie Coldstone, 
which has been kindly supplied to me by Dr Buchan. Further, 
the temperatures are the maximum temperatures for the day, and 
the humidities are the maximum humidities for the day calculated 
from the morning and evening readings considered with reference 
to the maximum temperature. The temperatures will therefore be 
I 1 higher than if they had been taken at the time the dust tests were 
made, and the wet bulb depression will also be much greater. 
We can therefore only compare these Alford tests amongst them- 
selves. It is, however, unnecessary here to enter into a detailed 
