44 THE MAMMOTH CAVE. 
the metaphor, it holds its breath—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 
