ORIGIN OF FOOD. 15 



AIR-CURRENTS. 



There is always a current of air in the cave. The current is variable 

 in strength, but always discernible. The air flows in at the mouth of 

 the cave and out through the cork-screw passage from October to March 

 and in the reverse direction from April to September. In the cork- 

 screw passage the current is very noticeable, as it is in all the narrower 

 parts of the cave. During February, 1904, I twice thought I detected 

 the air moving in part of the time and out part of the time during the 

 same day. But I noticed the apparent outward movement only when 

 in a larger portion of the cave, where the current is difficult to determine, 

 and was unable to be sure this fluctuation existed.* 



The cause of the currents is evident. During winter the cooler 

 outdoor air flows into the cave and displaces the warmer and lighter air, 

 which flows upward and outward through the cork-screw passage. 

 In summer the cooler air of the cave flows out at the mouth and the 

 warmer air enters the cave from above through the cork-screw. 



The air-currents tend to equalize the temperature within the different 

 parts of the cave and to bring it on a par with the temperature outside, 

 but the temperature of the surrounding earth exerts a sufficient influ- 

 ence to counteract largely the effects of air-currents, as well as of 

 the temperature outside the cave. The cool air flowing into the cave 

 in winter produces the decided change in temperature noted from 

 the entrance to the mound, radiation from the rock of the cave rapidly 

 warming the current all along, so that the influence is much reduced 

 toward the mound and is little felt at "42." When the current is in 

 the other direction, from April to September, the cave warms up in the 

 reverse order, so that the remotest portion is most quickly affected by 

 the current, while the portion from "17" to "6" does not reach its 

 normal temperature until midsummer. The constant outflowing of the 

 current during the warm seasons keeps the temperature near the mouth 

 very near the mean cave temperature, radiation from the outside having 

 little influence. 



ORIGIN OF FOOD. 



For food, cave animals are directly or indirectly dependent upon 

 organic matter carried into the cave from the outside, and they there- 

 fore lead a precarious existence. The supply of food is due to chance 

 and accident and is irregular in the extreme. 



*In Mammoth Cave, which apparently has but one opening to the exterior, the 

 air-currents, which are very strong, are variable, flowing sometimes in one direction 

 and sometimes in another, depending, it would seem, upon the temperature somewhat 

 and certainly upon barometric conditions. Should the outside opening into the cork- 

 screw passage of Mayfield's Cave become stopped with snow, fluctuating currents 

 might be expected in Mayfield's Cave. 



