90 Proceedings of Royal Society of Edinhurgh. [sess. 
Edinburgh and the surrounding towns, as they lie in that direction ; 
and the contracting of the lines to the S.W. is probably due to the 
impurities from Glasgow and the surrounding manufacturing towns, 
as they lie to the south-west of Ealkirk. The position of Edinburgh 
and Glasgow are shown on the map by small circles : the triangle 
to the N. W. represents the position of Ben Ledi. 
It will be observed that the isatmids, after passing the S.W., 
begin to open out and become much wider in a W.S.W. direction. 
This is probably an error due, as already stated, to the imperfect 
conditions under which the haze has to be estimated, making the 
W.S.W. winds appear clearer than they really are. The supposi- 
tion that most of the isatmid lines are too wide for W.S.W. winds 
is supported by the fact that the isatmid for 2° keeps close to 
Falkirk for Wk S.W. winds; the reason for this line keeping close 
being, that when the air is damp it is thick, and its haze is esti- 
mated in a short length of it. The estimate is thus made in the 
impure air itself, and the purer air in the distance has in this case 
no disturbing influence. 
It seems probable that if allowance were made for the conditions 
under which the estimates had to be made, the other lines would 
also keep close to Falkirk till after passing the W.S.W. direction, 
and then somewhat suddenly open out to the W. If this be so, 
then while the influence of Edinburgh and the towns surrounding 
it is shown by the crowding of the lines to the east of Falkirk, the 
influence of Glasgow and its neighbourhood is only imperfectly 
shown by the incurving of the lines to the south-west. From this 
it will be seen that the isatmid lines show the effects of the unequal 
distribution of population in even the thickly populated parts. 
The conclusions arrived at in this paper may be summed up as 
follows : — Accepting the two following conclusions, arrived at in 
previous communications on atmospheric dust, namely, that when 
the wind blows from populated areas the number of dust particles 
is always very great, and that for a given humidity the thickness of 
a haze is in proportion to the number of dust particles present, 
observations were made to test to what extent the air from popu- 
lated areas was hazed, compared with that blowing over thinly 
populated districts. 
Falkirk is so situated that the winds from the W., N.W. and N". 
