1892-93.] Mr Aitken on Hazing Effect of Atmospheric Dust. 85 
humidity, which it does not seem to do from the tables. But, 
before tables such as those given can show satisfactory results, it is 
evident we must have far more observations, in order to get the 
correct means. For instance, take the case of the south wind, the 
limit for 3° wet bulb depression rests on one observation, while that 
for 4° is the mean of four observations ; and so on all through the 
tables. Though 200 observations may be a fair number, yet, as 
there are sixteen directions of wind which should be entered under 
seven different degrees of humidity, — that is, 112 points to be deter- 
mined, — it is evident the observations are far too few to fix so many 
points. It would appear, however, that 200 observations, though 
far too few to give satisfactory results, yet when worked out they 
point to conclusions which agree fairly w'ell with each other, and 
with previous observations with the dust counter. 
It may be thought by some that the highest limit of visibility 
given in the tables is too great ; that 250 miles is too great a dis- 
tance for a mountain to be visible — that is, supposing it was above 
the horizon. This estimate has been made, as already explained, by 
estimating the haze on Ben Ledi, 25 miles distant from the place 
of observation ; and as this mountain was occasionally estimated to 
be only hazed, that gives 250 miles as the limit on these occa- 
sions. I may also add, that when observing on the Eigi Kulm I 
have often seen Hochgorrach so clear that it did not look more 
than 5 or 4 hazed. How, as that mountain is 70 miles distant 
from the Eigi, it makes the limit of visibility on those occasions 
about 300 miles. 
There is another and perhaps a more striking way of showing the 
results of these observations by means of what might be called 
isatmid* lines, . These are shown in the Plate which I have pre- 
pared for the purpose. As will be seen, these lines are drawn over 
a map of Scotland ; and to show the relation between these lines and 
the density of the population, the map is divided into counties, and 
the counties shaded according to the density of their population, 
the darkest being the counties in which there are most inhabitants 
per square mile. The density of the population is indicated by the 
scale on the map. These isatmid curves remind one of isothermal 
and isobaric lines. They are, however, drawn on quite different 
* Isos = equal ; = smoke, steam, vapour. 
