1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
215 
to have been 0 o, 9 C., or almost exactly the same as found by me. 
But as the thermometer at this situation is exposed in a metal box, 
it also would be exposed to radiation and slight heating. As there 
was a considerable amount of light at the time, there would be but 
little fog overhead. It seems therefore probable that radiant heat 
would penetrate and heat all surfaces and keep them dry, and heat 
the air in contact with /them. The wet bulb depression recorded 
was therefore probably too high. 
On the morning of the 22nd there were far more particles in the 
air than on the 21st, but the air was clear owing to an increase in 
the dryness, the wet bulb being now depressed 6°. As the day 
advanced the air got hazy, owing to an increase in both dust and 
moisture. On the 23rd the air was very clear, both dust and moisture 
having decreased. The dust and moisture remained much the 
same till the close of the observations, and the air retained its 
clearness to the end. So clear was it that occasionally Hochgerrach 
was visible, a condition of the atmosphere which, to those who know 
the Bigi, will be a good indication of the transparency. 
At Lucerne on the 25th the dust and moisture were both very 
low and the air was very clear. On the 26th the dust increased 
considerably, while the humidity remained nearly the same, and 
the air was hazy. 
Ho conclusions can be drawn on this point either from the Eiffel 
Tower or the London observations. 
Passing over the tests of city air, we shall now enter more into 
detail of the Kingairloch observations, as the air at that situation 
was less polluted, and the observations are better suited than the 
previous ones for showing the effect of dust and of humidity on 
the transparency of the atmosphere. First let us consider the effect 
which the number of particles has on the transparency of the air, 
and see if the conclusion contained in the previous paper is con- 
firmed by these later observations. This conclusion was that the 
greater the number of particles the thicker was the air. For this 
purpose, as already explained, we must select observations made on 
days when the humidity was the same. Taking those days on which 
the wet bulb was depressed 4°, namely, on the 6th, 10th, and 15th 
of the month, and arranging them in the order of the number of 
particles, thus — 
