540 Proceedings of the Royal Society of Edinburgh. [Sess. 
air coming from some of these directions was ten times more hazed than the 
air from the N.W. quadrant. From this it is evident we can only compare 
the air on the different days when the wind blows from the same direction 
and is of the same humidity. 
In these observations the things noted were : the temperature and wet- 
bulb depression, the direction and force of the wind, and the limit of 
visibility — that is, the most distant object, such as a hill, that was visible 
at the time. When the air was so clear that the most distant hill visible 
from the point of observation was quite distinct, then the haze on it was 
estimated. For instance, the most distant hill used in these observations 
is Ben Ledi, distant 25 miles. If this hill was just visible, the limit would 
be entered as 25 miles ; but if the hill was quite distinct, then the haze 
on it was estimated. If it appeared to be one-half obscured by the haze, 
the limit would be 50 miles. 
The first observations on haze were begun at Falkirk in June 1891, and 
continued to the end of 1892. Some 200 observations were made during 
that time, and the results are given in vol. xx., Proc. Roy. Soc. Edin. These 
observations were continued during 1893 and to near the end of 1894, and 
also during August 1896. Over 200 observations were made during this 
second period, but have not been published. They are now worked up 
and incorporated with the figures given in the first paper. Some of the 
observations were rejected owing to too high humidity or to there being 
too little wind, as high humidities and feeble circulation make observations 
of this kind unreliable. 
In working out the results of these haze observations, instead of giving 
all the readings the means are given in Table No. III. and the lowest and 
highest readings in Table No. IV. In preparing the observations for the 
tables the first thing to be done was to arrange them according to the 
direction of the wind at the time, all those taken when the wind was 
south being together, and all the others in a similar manner. Then the 
observations in each direction of wind were again separated according to 
the wet-bulb depression at the time, all those in which the wet-bulb 
depression was 2 degrees being together, then those in which it was 3 and 4 
degrees, and so on. When this was done, the mean limit of visibility 
of each lot was calculated. All these means are entered in Table No. III. 
There is also a parallel column in this table giving the number of 
observations of which the limit is the mean. It will be noticed that for 
all the winds the limit of visibility increases as the wet-bulb depression 
increases, and that all winds from N. to W. are very clear, while those 
coming from all other directions are more or less hazed ; those having an 
