202 Proceedings of Royal Society of Edinburgh. 
b 
a garden, and even here the number was small, but as the hotel is 
situated on the lake there was but little chance of pollution. 
These Lucerne and Yitznau observations showed the air of this 
part of Switzerland to be remarkably pure. We must, however, 
remember that on this occasion the wind was strong, and that it 
came from the wide unpolluted tract of country covered by the 
mountains and valleys of the Alps. On the day following these 
tests the wind changed and blew from the north-east, and brought 
up the air from the inhabited parts of Switzerland. The tests made 
on this day show a marked increase in the number of particles. 
Though the wind had only recently changed, the pure air of the 
previous day had acquired a great increase in dust in its passage 
through the inhabited parts. The number on the afternoon of this 
day varied from 2160 to 3660 per c.c. in the open country near 
Lucerne. 
The numbers obtained at all the Swiss stations, beginning with 
Locarno, are small, but the number of observations are too few to 
give anything like a true indication of the state of the air in that 
country. Still there are other reasons for supposing that the air of 
Switzerland is very free from dust. The vast tracts of mountainous 
country which surround it on most sides act as purifiers of the air, 
not only on account of the absence of population, and the time 
given for the dust to settle out of the air, but also by the pure 
upper air being mixed with the lower impure air, by the irregular 
currents and eddies formed by the wind in its passage across the 
mountains. It seems therefore possible that much of the trans- 
parency and brilliancy of the Swiss air may be due to its freedom 
from dust. 
The Eiffel Tower Observations. 
We have reason for supposing that the air at the top of mountains 
even such isolated ones as the Rigi, is not pure air, the same as 
one would get if they ascended to the same height in a balloon ; but 
that along with the pure air there is mixed much of the air of 
the valleys driven up the slopes by the wind or carried during 
calm weather by the air heated by the sun on the mountain slopes. 
The amount of this lower air which may be carried by the wind 
to the upper level depends on the shape of the hill and other 
