1892-93.] Mx kitVQii on Hazing Effect of Atmosplmnc Dust. 81 
the impure from the densely inhabited districts is often very 
marked. On one occasion when I was on the Denny Hills, situated 
to the west of Falkirk, while there was a westerly wind blowing, 
the view to the south through the impure air was limited to a few 
miles, while to the north it was so clear that the most distant moun- 
tains were seen through but little haze. 
When the air is thick, only the hill at four miles distance can he 
seen, then the Ochils become visible as the air clears, and at last 
Hen Ledi is seen when the haze becomes still less. Although this is 
not always the order in which the different hills become visible when 
the air clears, as has been already stated, yet it is the most frequent. 
After Ben Ledi is visible, it then becomes necessary to estimate the 
amount of haze on it, in order to get what we have called the limit 
of visibility of the air at the time. Thus, suppose Ben Ledi seemed 
to be half-hazed, then the limit of visibility will be 50 miles. In 
this way all the estimates of haze have been reduced to one scale for 
comparison. 
After going over all the observations entered in the note-book, and 
rejecting all those which were unsatisfactory, from the conditions 
being uncertain from want of wind, wet ground, showery weather, 
&c., there remained a little over 200 observations, which have been 
classified and arranged for the purpose we have at present in view. 
As already stated, if on two days the humidity was the same, but on 
one day the haze was thicker than the other, then we may conclude 
that the greater thickness was due to a greater amount of dust in the 
air. But if the air on one of the days was damper than on the 
other, then the increased density of the haze may have been due to 
the increased humidity. From this we see that we must compare 
the density of the haze only on those days when the humidity was 
the same. The first thing, therefore, which had to be done with the 
observations was to arrange them all in tables according to the wet 
bulb depression at the time. Tables were accordingly prepared, in one 
of which all the observations made when the wet bulb depression was 
2° were entered, while in other tables were entered the observations 
when the depressions were respectively 3°, 4°, 5°, 6°, 7°, and 8° and 
over. The different observations in each table were at the same 
time entered in such a manner that all those made when the wind 
was H. were together, all those when it was N.E. next each other, 
VOL. XX. 22/7/93 F 
