44 THE MAMMOTH CAVE. 



the metaphor, it holds its breath-r-when the 

 mercury in the thermometer stands at fifty-nine 

 degrees in the outer air, which is the average 

 temperature of all parts of the cave, winter and 

 summer. Hence it is frequently observed, in 

 the spring and fall, that there is no motion of 

 the air in either direction at the mouth of the 

 Cave. 



On entering the Cave a few hundred yards in 

 summer, when the temperature outside is at or 

 near one hundred degrees, the air rushes out 

 with such force as frequently to extinguish the 

 lamps. Passing into the Cave for about half a 

 mile, however, the motion of the air is barely 

 perceptible at any. time, from the fact that the 

 main avenue enlarges so rapidly that it plays 

 the part of a reservoir, where a current of air, 

 from any direction, is speedily neutralized. If 

 the current of air blows from without inward, 

 and is below fifty-nine degrees, it does not pass 

 more than a quarter of a mile before it is 

 brought up to that point. Air above the 

 average temperature of the Cave never blows 

 ' into it. 



.Thus it will be observed that a change of 

 seasons is unknown in the Mammoth Cave ; 

 and day and night, morning and evening, have 



