1$! Prof. Pictet on the Ice-caves of the Jura and the Alps. 
from the height of its mean temperature, which accords with the 
opinion of the inhabitants of the neighbourhood, that the hotter 
the season is, the greater is the cold, and the quantity of ice that 
is formed. However that may be, we had no reason to repent 
the trouble we had taken in this last expedition, as the grotto 
is in every respect much superior to the other two. We re- 
mained there till three o’clock in the afternoon, and returned to 
Geneva at eleven o’clock at night. 
After having stated the principal phenomena, and the various 
circumstances which fell under our immediate observation, in 
three different ice-caves, we shall now proceed to hazard some 
conjectures respecting their cause. 
We may remark, in the first place, That in two of the three 
ice-caves we visited (indeed we may say of the four, including 
that of La Baume), we cannot consider the winter’s snow as a 
possible or probable cause, although it might be a concomitant 
cause in the ice-cave of Brezon. Neither can we admit that 
the natural temperature of the earth favours these formations, 
for, in our latitudes, the mean temperature is far above the 
freezing point. Even oiir deepest springs indicate that it sel- 
dom falls below eight degrees. Besides, if this effect was pro- 
duced by the temperature of the earth, there would be ice found 
wherever there were natural caverns ; whereas, on the contrary, 
there exists a vast cavern without any ice very near the last ice- 
cave which we visited. 
It must be acknowledged, then, that we ought to have re^ * 
course to some particular local cause, in order to account for the 
formation of ice in some caverns and not in others. One of the 
observations made at the ice-cave of Brezon, puts us in the way 
of being able to solve this interesting problem : I mean that of 
the current of cold air, which issued with force from several 
openings in the rock near the cavern. Let us connect this fact 
with a class of phenomena, which have so much resemblance to 
those which have just occupied us, that the consideration of 
them may throw some light upon the causes which form and 
support the natural ice-caves. I mean those subterraneous ca- 
vities from whence issue in summer currents of . air, not only 
colder than the external air, but than the mean temperature of 
the soil in the regions where they are observed. 
