230 
Proceedings of Royal Society of Edinburgh. [sess. 
one frequently observes a great difference in the condition of the 
upper and lower airs ; and while making the observations on the 
transparency of the upper air which are entered in the table, a note 
was at the same time kept of the state of the lower air as it changed 
from hour to hour. As these latter observations bear directly on 
our subject, I shall here make some extracts from my note-book, 
and owing to the kindness of M. Billwiller of the Swiss Meteoro- 
logical Office, Zurich, I am enabled to give the condition of the air 
with regard to humidity at different places at the lower levels on 
the dates corresponding to the Rigi observations. 
Of the Swiss observations, those taken at Lucerne and Gersau are 
the most suitable for our purpose, as Lucerne is situated down the 
lake while Gersau is up the lake and on the opposite side of the 
Rigi. So that, while the Lucerne observations may be taken to 
represent the condition of the air towards the north of the Rigi, 
and the more open and inhabited parts of Switzerland, the Gersau 
ones will represent the condition of the air to the south and 
mountainous parts. 
On the 21st, owing to the fog, no view could be got of the lower 
air. On the morning of the 22nd the view was still closed by the 
stratum of clouds covering the country to the north. In the after- 
noon, however, these clouds cleared away sufficiently to show the 
condition of the lower air. In my notes it is recorded that the air 
high up was fairly clear ; but on looking down to lakes and valleys 
it was decidedly tliickisb, and in the evening the air to the north, 
east, and west was very thick, while to the south it did not thicken. 
Turning now to the Swiss Meteorological Report for this day, I find 
that at 1 p.m. the air to the north gave a depression of the wet bulb 
of 4°*2 C.,%hile that to the south gave 3°-0 C. But as the evening 
advanced the humidity at Lucerne increased, and at 9 p.m. the de- 
pression was only 1° C., while at Gersau the depression had increased 
to 7° ‘6 C. Here we have a clearly indicated relation between the 
transparency and the humidity. The observations on the hill top 
being made in the afternoon, the air to the north would be more 
nearly saturated than it was when the 1 p.m. readings were taken ; 
that is, the depression would be less than 4° C., and as stated the 
air looked thickish. In the evening it got very thick owing to the 
humidity increasing, the wet bulb depression decreasing to only 
1 
