78 
Proceedings of Royal Society of Edinburgh. [sess. 
given num'ber of particles, the damper the air the thicker is the 
haze. There are probably other causes of haze, but those referred 
to are the principal ones. 
The above conclusions place in our hands a means of comparing 
the amount of dusty impurity in different masses of air, or of 
different airs brought to us by winds from different directions. If 
we knew nothing about the.humidity of the air, observations on the 
thickness of the haze alone would tell us nothing. But if we know the 
humidity, we are a step nearer knowing the amount of dust. Tor 
instance, if we find that on two particular days the wind was from 
different directions, that the wet and dry bulb thermometers showed 
the same difference, but that the air from one direction was more 
hazed than that from the other, then we may conclude that in the air 
with most haze there was most dust. Though this method of working 
tells us nothing about the actual number of particles present in the 
different airs, yet it enables us to compare the relative purity of 
different masses of air, or of airs coming from different directions. 
It is further evident that by this method of working we must com- 
pare the haze only on days when the humidities are the same. It, 
however, seems possible, after we have accumulated a sufficient 
number of observations, that we may be able to estimate the effect 
of the humidity, and we may thus be enabled to compare days 
when the humidities are not equal. 
Adopting the above idea, observations were made to see if this 
method of working would give us any information as to what the 
hazing effect is of the products of combustion from the densely 
populated parts of our country. These observations were begun in 
June 1891, and have been continued to the end of 1892. The 
observations are not continuous, but were made from time to time 
as opportunity offered, and while I was at Talkirk, as all the obser- 
vations for this investigation must be made at one place. In the 
note-book in which the records were kept, the date and hour are 
entered in the first two columns. In the next two the direction and 
force of the wind, then the temperature of the wet and dry bulb 
thermometers, after which were three columns for entering the 
amount of haze on three hills at different distances, and last there 
was a column for remarks. 
For an investigation such as this Falkirk is particularly well situated. 
