1892 - 93 .] Mr Aitkeri on Hazing Effect of Atmospheric Dust. 77 
took considerable interest in the matter, the impurity of the air 
near Falkirk, and regretting the impossibility of making, near that 
place, any observations on dust which could be of any value from a 
meteorological point of view, owing to the vast amount of pollution 
thrown into the atmosphere by large towns such as Glasgow and 
Edinburgh, and the densely inhabited districts by which Falkirk is 
surrounded. My friend seemed somewhat astonished, and put the 
question — “ But is it possible that Glasgow smoke can travel all 
that distance, about 20 miles F’ To which the reply was, that there 
seemed to be no doubt about it, as the dust counter showed the air 
to be always very impure when the wind blew from densely in- 
habited areas. There was, however, always the possibility that the 
high numbers observed near Falkirk might have been due to local 
causes, though with the wind in some directions it could not have 
been so, as the area from S.E. to S.W. of Falkirk is very thinly 
populated for some miles southwards. This conversation turned my 
attention to devising means for testing whether the smoky impurities 
from towns and inhabited areas were carried to any great distance, 
and had any appreciable influence in thickening the air to the leeward 
of them. 
On considering this question it became evident that by making 
observations on the haze in the atmosphere at different times, and 
noting the direction of the wind and the humidity of the air at the 
time, an answer might be got without the use of a dust counter. 
Working in this way, all uncertainty as to the effects of the imme- 
diate local pollution would be got rid of, as these only affect the 
lower air, while the haze is measured higher up. 
In previous communications on the number of dust particles in 
the atmosphere, it has been shown that the thickness of a haze 
depends on the number of dust particles present, on the degree of 
saturation of the air, and to some degree also on the vapour pressure, 
— that is, on the absolute as well as on the relative humidity. 
Supposing we have two samples of air both of the same absolute 
and relative humidities, — that is, both at the same temperature, 
and both giving the same depression of the wet bulb thermometer, — 
if one of these samples be more hazed than the other, it will Be 
found to have more dust particles in it than the other, and to be 
the thicker the greater the number of particles present. And for a 
