The small town of Roquefort in France has been renowned ever since the time of the Romans for the delicious flavour of its cheese, which is said to owe its excellence to the cool cellars in which it is matured. These are excavated on the northern slope of a great chalk plateau, and communicate with numerous fissures in the rock, from which air-currents stream forth of so low a temperature as to cause a thermometer marking +23° R. in the shade, and in the external atmosphere, to fall to +4° R. when exposed to their influence. The cellars are so valuable that one, which cost 12,000 francs in construction, sold for 215,000 francs. In times of ignorance, superstition could not fail to attach its fables to the phenomenon of wind-grottoes. A cave near Eisenach was supposed to be the seat of purgatory, and popular credulity or terror willingly transformed the sounds produced by the rushing air-currents into the wailings of tormented souls. Fortunately, modern science affords us a more satisfactory explanation of the phenomenon. Pictet represents the case of a cave with cold currents of air to be much the same as that of a mine with a vertical shaft ending in a horizontal gallery, of which one extremity is in communication with the open air, at a point much lower, of course, than the upper extremity of the shaft. The cave or wind-hole corresponds to the horizontal gallery, and the various fissures in the rock take the place of the vertical shaft, and communicate freely with the external air. In summer the columns of air contained in these fissures assume nearly the temperature of the rock in which they rest—that is to say, the mean temperature of the district; and therefore they are heavier than the corresponding external columns of air which terminate at the mouth of the cave. The consequence is, that the heavy cool air descends from the fissures, and streams out into the cave, appearing as a cold current, and the hotter the day—that is, the lighter the columns of external air—the more violent will be the disturbance of equilibrium, and therefore the more palpable the current. The evaporation which takes place as the air-currents descend through the moist rock-fissures likewise tends to lower their temperature. Several naturalists have attempted to explain the phenomena of ice-caves in a similar manner, as being produced by cold currents still further refrigerated by the evaporation caused in the moist and porous rocks through which they pass. But to this theory there are weighty objections, as in many ice-caves there is no current whatever, and in all the cases of cold air streams investigated or mentioned by De Saussure, the lowest temperatures observed were still considerably above the freezing-point, and consequently incapable of converting water into ice.