AIR-CURRENTS IN CAVES. 7 



In Mammoth Cave a very perceptible air-current flows into the top of the dome 

 from Little Bat Avenue. It probably descends to the bottom of the dome and then 

 ascends at the side to flow out at Sparks Avenue. This current was flowing at 

 the rate of 8,640 feet per hour on November 30, 1902. It is probably caused by 

 a thin fall of water which descends from the roof of the dome to the bottom. 



By far the most violent air-current may be caused by a change in the atmos- 

 pheric pressure in the air without. These currents are perceptible only in caves 

 of considerable extent, and become violent when the opening is insignificant com- 

 pared with the size of the cave. 



When the weight of superincumbent air is lightened, the compressed air in the 

 cave expands and there is an outrush of air through the opening. If, on the other 

 hand, the barometric pressure increases when the superincumbent air column 

 gains in weight, there is an inrush of air. I have been at the entrance of Mammoth 

 Cave when the internal and external pressures were so equalized that the anemom- 

 eter would show ingoing and outgoing currents alternating irregularly every few 

 minutes. In 1902 I was also at the entrance x when the anemometer showed the 

 following rates per hour for air going in : November 29, 9 a. m., 46,350 feet ; 6 p. m., 

 39,840 feet; November 30, 7 a.m., 50,290 feet; 9 h 40™ a.m., 55,830 feet; and 

 i2 h 3o m p. m., 7,800 feet. 



Mr. A. M. Banta reports from Mammoth Cave that on January 31, 1903, "At 

 the gate the air-currents were surprisingly fitful. The current was running in 

 40 seconds, stopped 15 seconds, flowed out 8 seconds, stopped 10 seconds, and then 

 ran in for 2 minutes, when we left." His records give the following rates per hour 

 of air going in during February, 1903 : February 18, 12 m., 76,464 feet ; 5 h 3o m p. m., 

 77,396 feet; 6 h 20™ p.m., 79,896 feet; February 19, 10 a.m., 76,692 feet; 12 m., 

 68,904 feet; and February 21, 9 a. m., 56,556 feet. 



I know of no direct record of currents due to changing temperature on the 

 outside. Until direct observation with an anemometer had been made the general 

 impression among the guides at Mammoth Cave was that air rushed in during one 

 part of the year and out during the other. On cold winter days at Mitchell frost on 

 the bushes showed that a gentle current of the damp cave air was flowing out from 

 the upper part of the cave. The strength of the convection currents is undoubtedly 

 dependent in large measure upon the shape of the cave and the nature of the open- 

 ing. But the influence of water-currents or winds might at any time be sufficient 

 to change the direction of the convection currents. 



Nothing very definite can be said about the size of the environment afforded 

 by a cave. 2 While it is known that some caves are much larger than others, it is 

 never certain how large the unexplored or unexplorable part of a cave may be, how 

 far the smaller cracks lead, and in how far they may establish intercommunica- 

 tion between neighboring caves. 



1 A wall partially closes the entrance avenue so that the air passes in and out through a narrow gate where 

 the currents were measured. 



2 Hovey (The Mammoth Cave of Kentucky 1897, p. 64) makes the longest course in Mammoth Cave 

 from the entrance to Grogham Hall about 4.5 miles; the total length of all the known channels is several times 

 that. The width and height may vary greatly from the many cracks where one has to crawl to Chief City between 

 450 feet (Hovey) to 541 feet (Call) long, and an average width of 175 feet (Hovey) to 190 (Call), with a maximum 

 width of 287 feet. 



Blatchley says of Marengo (p. 157), "Marengo Cave has been advertised far and near as containing 7 miles 

 of underground passages. Our measurements showed its total length to be 3,850 feet, or 0.7 of one mile. The 

 main channels of Wyandotte Cave we determined to be 4.21 miles long." Very many of the caves are but a few 

 inches in diameter and too small to be entered. 



