232 Proceedings of Royal Society of Edinburgh. [sess. i 
and south of the mountain may have partly been due to the amount 
of dust in it, and the blackness noticed on the 23rd may have been 
due to this cause, as at Lucerne the wind in the early morning of 
that day was blowing from the north, and would bring polluted air 
up from the populated parts of the country. This north wind did 
not seem to penetrate into the mountainous parts south of the Rigi, 
as the wind at Gersau was south-west. 
It may he asked, Is the air at the top of a mountain polluted to 
any extent by the valley air ? Is the air on the mountain top the 
same as we breathe in the valley, only reduced in pressure? ~No 
very direct answer can he given to this question. There seems 
however, to he very little doubt that the valley air frequently 
rises to the tops of hills, hut this valley air will generally he more 
or less mixed with the purer upper air, the amount of impure air 
depending on the height, the shape of the hill, and other causes. 
It will be observed from the observations made on the 25 th of 
May on the Rigi Kulm and at Yitznau at the foot of the mountain, 
that there was no very great difference in the amount of dust in 
the air at the two places. When allowance is made for the higher 
pressure at the lower station, there was less at the foot at midday 
than at the top in the morning. For practical purposes we may 
look on the dust on these two occasions as being the same, showing 
that in all probability the air tested at the foot of the mountain 
was the same as at the top. This, however, does not prove that the 
air of the valley ascended to the mountain top, because on this 
occasion a strong wind was blowing and the air was coming from 
the unpolluted area of the Alps, so that both the upper and lower 
air might have been free from local impurities. The observations 
made on Callievar showed there was much less dust in the air at 
the top than at the foot ; still they also showed that the air came 
up from the valley as the numbers increased with a change of wind 
from a populated direction. The observations made on Finouillet 
show the air at that station to he frequently very impure from the 
ascent of valley air. The Rigi observations also show that in all 
probability the valley air came to the hill top of course more or 
less diluted with pure air. When there was little wind, as on the 
21st, the amount of dust was small; but on the 23rd, when the 
wind began to blow, the amount of dust gradually increased to a 
