84 Proceedings of Royal Soeiety of Edinhurgh. [sess. 
to have divided them into different series according to the reading 
of the dry bulb thermometer at the time, because it has been shown 
in previous communications that there is good reason for supposing 
that for a given wet bulb depression the hazing is greater at high 
than at low temperatures. The observations ought, therefore, to 
have been arranged in tables according to the temperature at the 
time. But as the number of observations are so few, it was thought 
better to treat them in one series, as if temperature had no effect. 
Table ISTo. I. gives only the mean limit of visibility of a number 
of observations for each direction of wind ; it may therefore be 
interesting to show how much the individual observations differed 
from each other. Bor this purpose Table No. III. is given. In the 
vertical columns, under the different wet bulb depressions, are entered 
the least and greatest limits of visibility observed for the different 
winds entered in the first column. This and the other tables are 
very incomplete, owing to there being no observations of some 
winds at certain degrees of humidity. It will be noticed from 
Table III. that the limit varies considerably for the same wind at 
the same humidity. This is what might have been expected, because 
it has already been shown by the observations with the dust counter 
that the number of particles varies greatly in winds from the same 
direction but at different times. As this variation takes place even 
in winds coming from what we call unpolluted areas, it will also 
take place in that coming from inhabited districts, owing to rise and 
fall of wind, changes in the state of trade, season of the year, &c., 
requiring more or less coal to be burned at one time than another. 
The limits given in Table III. do not seem to be greater than what 
might have been expected, nor are they greater than the dust 
counter has shown the number of particles to vary. 
As already stated, the limit of visibility increases as the air gets 
drier. This is seen when we look along any of the horizontal lines 
of figures given in Table No. I. In Table No. II., where the 
observations are still further condensed, the mean limit of visibility 
rises rather more regularly with the decrease in the humidity than 
in Table No. I. It will be further noticed that, as a general result, 
the transparency of the air increases about 3*7 times for an increase 
in dryness from 2° to 8° of wet bulb depression. The limit of visi- 
bility ought probably to rise regularly with the decrease in the 
