4MMAMMOTH CAVE.



the end of Dickson's Cave, can be distinctly heard
at the entrance of Mammoth Cave.
  The present entrance to Mammoth Cave was form
ed, and its communication with Dickson's Cave cut
off by the disintegrating action of the water of the
spring, which discharges its contents from the ceiling,
at the mouth of the former, and which caused the
Cave at this point to fall in-thus establishing a new
entrance, and shortening the length of the Cave by
about half a mile. Dickson's Cave differs little in
size and appearance from Proctor's Arcade, in the
M1ammoth Cave.
         RESPIRATION OF THE CAVE.
  The Mammoth Cave breathes once a year. That
is to say, in summer, or when the temperature of the
external air is above that of the Cave, the current
sets from the latter to the former. In other woIrds,
the Cave is the entire summer in making an expira-
tion. On the other hand, when the order is reversed,
or the temperature of the outer atmosphere is below
fifty nine degrees, the Cave makes an inspiration, or
draws in its breath which it a'complishes during
the winter. The respuratory mceharissn of the Cave
ceases to operate, or, to carry out the metaphor-it
holds its breath-when the nreercurg in the ther'mom-
eter stands at fifty nine degrees in the outer air;
which is the average temperatur,-e f all parts of the
Cave, winter and summer   Hensce ia, is frequently
observed, in the spring and fall, that there is no
mnotion of air in eitheir direction at the mouth of
the Cave.
  On entering the Cave for a few hundred yards, in
summer, when the temperature is at or near 100 de-
grees, the air rushes out with such force as frequently
to extinguish the lamps. Passing into the Cave for
about a half mile, however, the motion of air is
barely perceptible at any time, from the fact that the
main avenue enlarges so rapidly that it plays the



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