1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
243 
in an interesting way in the Alford observations. For some days 
towards the end of the observations it will he noticed that the amount 
of dust was small, and the night minimum steadily decreased, 
while the small amount of dust continued. But on the 16th, the 
last day the tests were made, when the sudden increase of dust took 
place, there was a change also in the minimum temperature, which, 
instead of falling to a lower point, was the following night higher 
by nearly 12°. We must not, however, conclude that these 
changes of night temperature were entirely due to dust, as it will 
he noticed that the wind was occasionally northerly while the 
low night temperatures lasted, and that on the 15th the direction 
changed to southerly. 
In the previous remarks there is something like a contradiction, 
which it may he as well to explain here. It is stated that the wind 
decreases the amount of dust, and also that it raises the minimum 
temperature. It is further stated that a decrease in dust is accom- 
panied by a low minimum temperature. This apparent contra- 
1 diction disappears when we consider that it is only when the wind 
has fallen that we can see the effect of the low dust in producing 
a low temperature. 
We might sum up the results thus : — 1st, During the summer 
there was generally a high maximum temperature when there was 
a maximum of dust in the air. 2nd, There was a tendency to a 
I low temperature at night when there was little dust in the air. 
3rd, During the time of the Dumfries observations the mean tem- 
perature was high, and the amount of dust great. 
Stated in another way, it amounts to this, that dust increases 
the day temperature and checks the fall at night, thus increasing 
the mean temperature. If this be so, then it looks as if the dust 
absorbs the sun’s heat during the day, but does not radiate the 
earth’s heat at night. We might expect that the condition of air 
that got highly heated by the sun’s rays would be greatly cooled 
at night by radiation. It will be as well, however, to remember 
that at present we are in great ignorance as to the diathermancy 
and radiating power of our atmosphere, and further it is easy to see 
j that the heating and cooling will take place under different condi- 
tions. The sun’s rays heat the air, and evaporate the moisture 
from the dust particles, and so tend to open the atmosphere to the 
