16 Prof. Pictet on the Ice-caves of the Jura and the Alps. 
cavity through which it has passed, but all the refrigerating 
effect of evaporation, which must be very considerable, if the 
space traversed is filled with fragments of stone, constantly wa- 
tered by some little stream. 
Such is the most plausible explanation of the cold winds 
which issue from the grottoes at the foot of Monte Testaceo, 
and of all the cold caves we have mentioned, and which (it 
must be recollected) are always placed against some mountain 
or hill. 
Saussure, who suggested this ingenious explanation, wished 
to ascertain the refrigerating effect which it supposes, by an ex- 
periment, which, if it did not shew the highest possible degree, 
at least proved the reality of it. He filled a glass tube of an 
inch diameter with fragments of wet stones, and made the air 
rush through by means of bellows ; the air, which was at 7£° 
at its entrance, was at 65° f when it came out of the tube. He 
afterwards obtained an additional degree of cold, by directing 
the bellows against the ball of a thermometer wrapped up in 
wet linen. 
If we suppose the mean temperature of the cavity in which the 
air thus circulates, to be at 48° or 50°, it will not appear very ex- 
traordinary that the refrigerating process of evaporation, favour- 
ed by a great multiplication in the surfaces, and by a tempera- 
ture already cooled by the evaporating water, should attain the 
freezing point, and in this case the natural ice-caves are only 
grottoes, with cold currents of air, which are a little colder than 
others, because the refrigerating effect of evaporation is there 
more favoured by circumstances. 66 This explanation of cold 
currents,” says Saussure, “ has nothing forced in it, at least for 
countries near our Alps, which are those where the coldest caves 
have been observed.” We saw the thermometer at 37°| in the 
caves of Caprino, and at 38°f in those of Hergisweil, in the mid- 
dle of summer. There is not a very great difference between this 
and the temperature of 48°, which we observed in the ice-cave of 
St Georges, or of 33° in that of Mont Vergy. It must be ob- 
served, that this theory explains very naturally the extraordi- 
nary fact, attested by all the inhabitants near the ice-caves, that 
there is more ice formed in summer than in winter. The rea- 
son is a simple one : it is in summer $ and in the hottest part of 
