6 BLIND VERTEBRATES AND THEIR EYES. 



Conditions of moisture, while practically uniform in some parts of caves, fluc- 

 tuate in others more than any other element of environment. The maximum degree 

 of moisture is naturally found in the pools and streams. On the other hand, in 

 the upper parts of Mammoth and Wyandotte Caves the dust lies undisturbed for 

 years. In Mammoth Cave the tracks of oxen made in i860 are now shown to 

 visitors, and I am told that in Wyandotte the still older tracks of the moccasined 

 Indians are perceptible to-day. There are, however, parts of caves where the 

 moisture dripping through from above is considerably increased after a rain, and 

 the River Styx in Mammoth Cave rises 60 feet above low-water mark. The creek 

 in Shawnee Cave sometimes fills parts of the cave to the ceiling. 



The conditions of the water also change very greatly. At ordinary times it 

 may be very clear ; after rain it may carry a large amount of sediment. In its low 

 condition it may flow very quietly, in its high condition be a torrent. The water, 

 then, fluctuates in amount, clearness, and swiftness, with meteoric conditions. 



Charts of simultaneous records on two self-registering barometers show the close 

 agreement in changing barometric pressures inside a cave and outside it. One of 

 the instruments was placed about 90 feet above the exit of the cave, the other near 

 the middle of Shawnee Cave. Records chosen on account of peculiarities in the 

 rise and fall of the pressure at certain times leave no room for doubt that baro- 

 metric changes similar to those of the outside take place in the caves. 



The following table shows the temperatures for air and water in Donaldson 

 and Shawnee Caves in 1906 and 1907 : 



' The higher temperatures are caused by rains and last only a few hours after a heavy rain. During the first 10 days in September, 1907 , 

 the temperature of the water was 14.5, 15.6, 17.3, 16, 14-9. 14-6. i3-9, and 15.3 on successive days. During the last 10 days of the month it 

 ranged from 15 to 15. 5°. 



a From the 1st to the 15th the temperature was between 10.6 and 11.6. 



Currents in water and air differ materially in different caves and at times in 

 the same cave. In the Cuban blind-fish caves there is neither appreciable air- 

 current nor water-current, so that the evaporation from the quiet surface of the 

 water forms a covering crust of carbonate of lime and magnesium. In the blind-fish 

 caves at Mitchell, Indiana, a small current of water flows during normal conditions. 

 The stream becomes a raging torrent in high water. 



Currents in the air may be caused, (1) by the flow of water ; (2) by the epigean 

 air-currents ; (3) by changes in the atmospheric pressure ; and (4) by differences 

 in temperature. 1 



1 A detailed study of the currents of air and temperature of the water in the Mitchell Caves will be published 

 within a year. 



