220 Proceedings of Royal Society of Edinburgh. [sess. 
conclusions, however, require many more, and much more carefully 
conducted experiments to prove them satisfactorily. The objections | 
to the Dumfries observations are many. First, the situation is low, 
and the dust would tend to collect there to an undue amount. 
Second, the humidity is too high, owing to this station being near 
water, so that while the dust-counter and the thermometers gave 
the condition of the air in the valley, the tests for transparency 
were necessarily made through a higher stratum of air, in which 
both dust and humidity would probably be less. Then again, the 
difficulty of estimating transparency, and also in comparing these 
estimates made at different stations. The different hills visible at 
each station should have been graduated off, and used as a scale of 
transparency. This, however, was not done till the last observations 
were made. 
Though these results should be received with caution, yet I think 
it will be admitted that they clearly point to an interesting line of 
enquiry, and encourage us to push on our investigations into the 
dust in the atmosphere. I may remark that it is advisable not to 
make comparisons of these tests when the wet bulb is depressed only 
about 1°, because when the saturation is so great, accuracy can- 
not be attained, as local conditions, such as wet trees, the exposure 
of the bulbs, &c., may cause an error equal to the whole depression. 
The Dumfries observations show an interesting relation between 
the weather and the amount of dust in the air. During the whole 
time the weather remained dull and thick, the air was highly charged 
with dust. Another important point was that when the clouds 
cleared away, dense fogs formed, owing probably to the great 
radiating power of the air, due to the great amount of dust in it. It 
will also be noticed that on the fine days the air had generally little 
dust in it. 
The conclusion forced upon us by a consideration of all these 
observations is that the dust in our atmosphere condenses vapour 
when the air is far from being saturated. It seems probable that 
in all states of humidity the atmospheric dust has some moisture 
attached to it, and that as the humidity increases the load of 
moisture also increases ; and further, all observation seems to point 
to this form of condensation producing even a very thick condition 
of the atmosphere before the air is quite saturated. The observa- 
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