250 PHYSICAL GEOGRAPHY. 



raised, the cooler air of the autumn and of the winter begins to penetrate ; but before 

 this lower temperature can establish itself, it is again overtaken by the warmth of the fol- 

 lowing spring and summer. The consequence of all this is a temperature subject to little 

 alteration, but lower than the mean temperature of the surrounding atmosphere. The 

 ancient Romans, hence, according to Seneca, were accustomed to erect their country 

 seats in the vicinity of those natural cavities which abound about the capital, for the 

 purpose of enjoying their refreshing chilliness in the summer season. It was in one of 

 these volcanic caverns that Tiberius was nearly destroyed while at supper ; for, during 

 the banquet, the roof suddenly gave way, and buried several of his attendants beneath 

 its ruins, when Sejanus threw himself upon the emperor to preserve him from the falling 

 stones. 



Many caverns, however, vary greatly in their temperature, and exhibit the appa- 

 rently strange anomaly of being cold when the external air is warm, and warm when it 

 is cold, in some instances carrying this contrast to the extreme, so as to be coated with 

 ice amid the heat of summer, and affording a comfortable warmth amid the cold of winter. 

 In the neighbourhood of Czilicze, a village of Hungary, there is a cave in the transition 

 limestone of the Carpathians which displays this phenomenon. The country in the 

 vicinity abounds with woods, and the air is sharp and cold. The entrance of the cavern, 

 which fronts the north, is eighteen fathoms high, and eight broad ; consequently, wide 

 enough to receive a large supply of external air, which, here generally blows with great 

 violence ; but the subterranean passages, which consist entirely of solid rock, winding 

 round, stretch away farther to the north than has been yet discovered. In the midst of 

 winter the air in this cavern is warm ; but in summer, when the heat of the sun without 

 is scarcely supportable, the cold within is not only very piercing, but so intense that the 

 roof is covered with icicles of great size, which, spreading into ramifications, form very 

 grotesque figures. When the snow melts, in spring, the inside of the cave, where its 

 surface roof is exposed to the sun, emits a pellucid water, which immediately congeals as 

 it drops, and thus forms the above icicles, and the very water that drops from them on 

 the sandy ground freezes in an instant. It is even observed that the greater the heat 

 is without, the more intense is the cold within, so that in the dog days all parts of this 

 cavern are covered with ice, which the inhabitants use for cooling their liquors. The 

 quantity of ice is so great that a narrator estimates that it would require six hundred 

 waggons to remove it in a week. In autumn, when the nights grow cold and the heat 

 of the day begins to abate, the ice in the cavern begins to dissolve, so that by winter 

 no more ice is seen. The cavern then becomes perfectly dry, and has a mild warmth. 

 At that season it is the haunt of swarms of flies, gnats, bats, owls, and even of hares 

 and foxes that resort hither, as to their winter retreat, and remain till the return of 

 spring. An instance almost as singular occurs at Besan9on, in a grotto which extends 

 364 feet into the rock, the mouth of which, like that of Czilicze, is towards the north, 

 and covered with vegetation. During the whole summer this cavern contains masses of 

 ice, which melt away in October and November. 



This apparently anomalous phenomenon is susceptible of a very satisfactory explanation 

 by the relation which subsists between the moisture in these caverns and the external air. 

 When it is hot and dry outside, as in summer, evaporation takes place, and by this means a 

 considerable degree of warmth is withdrawn from the enclosed air, the vapours making their 

 escape through the openings, and through fissures in the roofs. The greater the exterior 

 temperature the more vigorously the evaporation is carried on, producing a degree of cold 

 in the interior which may sink beneath the freezing point, just as in the greatest heat we 

 can most readily freeze water if we surrround it with ether. It is upon this principle 

 that travellers, in some regions, are accustomed to cool their drinks, which they bury in 



