Prof. Pictet on the Ice-caves of the Jura and the Alps. 15 
“ without doubt, (says Saussure,) this is produced by the cur- 
rent which enters into these subterranean cavities.” 
Further on he adds, “ The proprietors of these cold caves 
unanimously affirmed, that the hotter the summer was, the 
greater was the strength of the current of air which issued 
from them, and which re-enters in winter from the intensity of 
the cold.” 
We shall also add to this phenomenon another fact equally 
authenticated, and which points out the cause of the first very 
distinctly. We know, that in working mines, situated on the 
declivity of a mountain, after having dug a vertical pit, they 
join it at the bottom to a horizontal gallery, which opens out to 
the air. These two cavities, which form a right angle, one 
branch of which is vertical and the other horizontal, act as a 
syphon, from which the air issues in summer, from the lower 
orifice, and rushes in from the top to the bottom of the pit; 
whilst in winter, it enters, on the contrary, by the gallery, and 
forms in the pit an ascending current. These two currents, 
whose direction is inverse, are produced by the different rela- 
tive weight of two currents of air, which balance reciprocally the 
one in the pit, the other in the open air, and which have for their 
base the door of the gallery, and for their common height the 
depth of the pit. 
In summer, the exterior column yields, because being hotter, 
it is higher than the interior one ; and the greater the rupture 
of the equilibrium of temperature between the two columns, 
with so much the greater force does the interior column descend 
and issue from the gallery. In winter, the rupture takes place 
in the opposite manner. The column in the pit is warmer than 
the external one, and is pushed up to the top, from below, by 
the latter, which rushes into the gallery. And, in the inter- 
mediate seasons, when the temperature without and within is 
equal, then there is no movement in the air either way. 
There is little reason to doubt, that the same alternations take 
place, wherever there exist natural cavities, nearly in a vertical 
direction, terminated below by lateral openings in the form of 
grottoes. A current of air must be established, which, in sum- 
mer, will be from above to below. This current brings into the 
grotto not only the temperature of the vertical portion of the 
