62 CARL H. EIGENMANN. 



3. A feature no less striking, though varying greatly in dif- 

 ferent caves, is the constancy of the temperature - - the absence 

 of great diurnal and seasonal changes. This differs very much 

 in different caves, and we have caves in the temperate zone in 

 which there is always ice - - in summer as well as in winter 

 and others in which ice never forms. This difference is due to 

 the shape of the cave and the accessibility of outside air in winter 

 and summer. Usually seasonal changes are very slight. 



4. This unit of environment is further characterized by the 

 relatively stationary atmosphere, the absence of rain, snow, or 

 severe currents of air. In the deeper recesses of a cave it is 

 rarely that an air-current is perceptible with an anemometer 

 unless there is moving water near. In the blind-fish caves of 

 Mitchell, Indiana, a current of air enters the cave above with the 

 water and leaves it three quarters of a mile below with the 

 water. In Little Bat avenue, one of the upper tiers of river chan- 

 nels of Mammoth Cave, there is no perceptible air-current until 

 the top of Mammoth dome is approached, where 'a perceptible 

 current enters the dome, descending to the bottom of the dome 

 and then ascending again to leave the dome about half way up 

 by Sparks avenue. This local air-current is caused by the cas- 

 cade descending from the uppermost to the lowermost part of the 

 dome. 



Air-currents are most readily perceived at a cave's entrance 

 and vary with the size of the opening, the size of the cave and 

 the rapidity of barometic changes on the outside. 



In a small cave air- currents are not strong about the entrance. 

 In a large cave, such as Mammoth, where the opening is small 

 the current may become a fierce gale if barometric changes are 

 rapid or if the water in the cave rises rapidly. If the barometric 

 pressure increases there will be an inflow of air and this will be 

 proportionate to the rise of the barometer and to the size of the 

 cave. If the barometer is falling there will be an outpour of air 

 and the pressure on the inside or outside may be so nicely 

 balanced that no current will be perceptible, or there may be a 

 continued shifting of currents in or out every few moments with 

 every slight shifting of outer atmospheric pressure not perceptible 

 with the ordinary mercurial barometer. 



