1889-90.] 
Mr J. Aitken on Dust Particles. 
241 
temperature apart from its effect as a vapour, as a high humidity 
will increase the effect of the dust by increasing the size of the 
particles, while a low humidity will decrease the influence. The 
records of observation are as yet too fragmentary to warrant the 
drawing of any general conclusions on this subject. 
Before the relation between the dust and the temperature can he 
settled with any degree of certainty, far more data are required. 
We would require to have, along with the dust tests, records of the 
strength of the sunshine taken by a more accurate method than the 
ordinary black-bulb vacuum thermometer ; and also records of the 
hours of sunshine, the direction and velocity of the wind, not made 
at long intervals but continuously. 
We shall now turn to the diagrams, and see if the earth’s radia- 
tion at night is influenced by the amount of dust in the air, but 
our information is too meagre for a correct interpretation of the varia- 
tions in temperature. The fall of temperature at night being due 
to the earth’s radiation, before we can compare the cooling on 
different nights, continuous records of the amount of cloud in the 
sky are required, as well as the direction and velocity of the wind. 
As no records were kept of the clouds at night, and only imperfect 
ones of the wind, any conclusion we may draw from these curves 
will be of little value. The Dumfries observations will be most 
suitable for studying the night radiations, as they were taken during 
the long nights of winter. At first sight there does not seem to be 
any relation between the amount of dust and the cooling taking place 
at night. The reason for this is, that there are other influences at 
work besides dust, and one of these is the velocity of the wind — a 
high velocity preventing an accumulation of cold air near the earth’s 
surface by mixing the cooled air with the warmer upper air. The 
Dumfries observations show that on all nights when there was wind 
the fall in temperature was slight, and that great falls only took place 
in calm weather. To get the effect of dust we must therefore com- 
pare only nights on which there was no wind or only slight airs. In 
order to assist our inquiry, there is placed in the diagram of the 
Dumfries observations another series of temperature observations 
taken by means of a minimum thermometer placed on the grass. 
These observations are shown connected by the curve underneath 
the curve of minimum air temperature. The difference between 
VOL. XVII. 26/6/90 Q 
