1892 - 93 .] Mr Aitken on Hazing Effect of Atmospheric Dust. 79 
By examining a map of Scotland, it will be seen that Falkirk lies a 
little to the north of a line drawn between Edinburgh and Glasgow, 
and is nearly midway between them. If we draw a line due west 
from Falkirk, and another due north, we shall find that in the north- 
west quadrant so inclosed the population of that part of Scotland is 
extremely thin, the country over that area being chiefly mountain- 
ous, and there is not a town in it of any size within 70 miles, with 
the exception of Stirling. In all other directions outside the N.W. 
quadrant the conditions are quite different. In the north-east quad- 
rant are the fairly well populated areas of Aberdeenshire, Forfarshire, 
and the thickly populated counties of Fife and Clackmannan. In 
the south-east quadrant are situated Edinburgh and the well popu- 
lated districts of the south-east of Scotland, and in the south-west 
quadrant are Glasgow and the large manufacturing towns which 
surround it. It will be thus seen that Falkirk is surrounded on 
three out of the four directions by thickly populated areas, while 
the fourth is very thinly populated. The result of this is, that 
while the winds from the N.E., S.E., and S.W. quadrants bring us 
air polluted in its passage over populated areas, the winds from 
the N.W. quadrant come to us comparatively pure. If, now, we 
compare the air that comes to us from those different directions in 
the manner already described, we shall see what the effect is of the 
products of combustion on the clearness of our atmosphere. 
While Falkirk is particularly well situated with regard to its posi- 
tion relatively to the pure and impure surroundings, being situated 
at the apex of the pure hf.W. quadrant, it is yet rather unfortu- 
nately situated with regard to the conditions necessary for esti- 
mating the amount of haze in the atmosphere. As in previous 
observations, the haze has been estimated by noting the most distant 
hill that could be seen through the haze. The distance in miles of 
the furthest away hill visible is then called the “ limit of visibility ” 
of the air at the time. But as it is almost never possible to get a 
sufficient number of hills at different distances to work in this way, 
the limit of visibility has generally to be arrived at by estimating 
the amount of haze on some hill at a known distance, and calculating 
from that estimate the greatest distance at which a hill could be 
seen under the conditions, if such a hill were in existence. For the 
observations made at Falkirk only three hills are available : one a 
