541 
1909-10.] Halley’s Comet and the Earth’s Atmosphere. 
easterly component are the worst, being at all degrees of humidity only 
about one-tenth as clear as the north-westerlys. 
Table No. IV. shows bow variable the limit is in the different observa- 
tions in winds from the same direction and with the same humidity. 
Part of this variability is due to the difficulty of estimating haze. For 
instance, if the sun is in front and near the line of sight the haze is much 
denser than when the sun is behind. Then the background of the distant 
hill has a great effect when estimating the haze. If backed by shaded 
clouds, which I consider to be standard condition, it seems much more 
hazed than when backed by a clear bright sky or a setting sun, against 
which it gets silhouetted ; and estimates made under these conditions are 
not trustworthy, or must be allowed for. Another thing which causes 
difficulty is the want of homogeneity in the air. I have stood on a hill 
about five miles to the west of Falkirk when a W.S.W. wind was blowing. 
Looking towards the south the limit of visibility was only a mile or two, 
while looking towards the north-west the air was very clear, limit being 
about 200 miles. At the time referred to I happened to stand on the 
dividing line, which is here very sharp, between the populated and un- 
populated areas, and along this line the wind was blowing. All south of 
this line is a busy manufacturing area, and all north of west is pasture 
and moorland. Supposing, now, that the W.S.W. wind then blowing 
changed to S.W., this would bring some miles of the thick air between me 
and Ben Ledi, while beyond that thick air there would be pure air, and 
in entering the observation it would go down under S.W. air, while the 
estimate would be made only partly in that air and mostly in purer air. 
Now, from this it is evident that all estimates of haze should be done 
either up or down wind, and when possible with the back to the sun. 
But as there are few situations giving an all-round view to a great 
distance, we have to be content with and make the most of what is within 
our range. Difficulty in estimating haze is also caused by the air at low 
level not always having the same transparency as that at the level of the 
top of the mountain. 
It was thought that if the tail of the comet had any effect on our 
atmosphere, either by increasing the dusty contents or otherwise, we 
might expect to find some evidence of it in the haze in the lower air 
some time after the passage of the comet. I therefore began taking 
observations on haze at Falkirk after my return from Morar. These 
observations were interrupted while I was observing at Appin, but were 
started again on my return and continued till the 24th July. They 
have been treated in the same way as the earlier ones, and the mean 
