Mammoth Cave of Kentucky. 229 



only one current existed, and that this flowed out when tlie external 

 air was above 60*, and inward, when this was below 60°. Going 

 in, one day at noon, we found the outward blast very strong : we 

 prolonged our stay until past midnight; meanwhile, a storm of rain 

 accompanied by lightning had come up, and at three a.m., when we 

 again emerged, the temperature outside had fallen to 50°, and the 

 inward gale blew so strongly, as to extinguish our lights, several 

 hundred yards from the mouth. In fact, the guide told us, when 

 more than two miles in the cave, that a change had taken place in 

 the outer air, and that we should probably find a storm raging with- 

 out. His accustomed senses detected the gentle current inward, 

 which we did not notice at so great a distance, and he perceived, as 

 he afterwards told us, a change of level in the subterranean rivers 

 since our crossing them in the morning, — the rain which had fallen 

 copiously having already af!ected them. 



I was at a loss for a little time to account for these currents of 

 air, but the following explanation suggested itself: — The mouth of 

 the cave is the only communication between the external air and the 

 vast labyrinth of galleries and avenues which stretch away for many 

 miles in the solid limestone. The extent of these under-ground 

 valleys of denudation, or rather " excavations," is vastly greater 

 than I had imagined before going there ; branching, crossing, inos- 

 culating in all directions, and at all levels, the mind is lost in amaze- 

 ment at their vast proportions, the various beauty which they possess, 

 and at the continuity of the cause which has formed them. The air 

 which they contain is pure and exhilarating. The nitre beds, which 

 are of incredible extent in these galleries, probably account in part 

 for the purity of the air, as the nitrogen, which is consumed in the 

 formation of the nitrate of lime, must have its proportions of free 

 oxygon disengaged, thus enriching this subterranean atmosphere 

 with a large portion of the exhilarating principle. The temperature 

 of the cave is uniformly 69° Fahr., summer and winter, and this is 

 probably very near to the annual mean of the external air. The 

 expansion which accompanies an elevation of temperature in the outer 

 air is immediately felt by the dense air of the cave, and it flows out 

 in obedience to the law of motion in fluids, and the outward current 

 continues without interruption, as long as the outer air is possessed 

 of a higher temperature than the cave. 



I was assured by those resident on the spot, that, for weeks or 

 even months, in the summer and winter, this blast of air continues 

 with unabated force outward or inward, and yet the thermometer, at 

 a distancQ from the mouth, always indicates the same temperature. 

 This seems to me of itself a very convincing proof of the vast volume 

 of the subterranean galleries ; for otherwise the equilibrium being 

 restored, the current would cease, or the temperature within would 

 vary. Still it must bo regarded as an interesting place for a series 

 of observations with the barometer and thermometer, continued daily 



VOL. LI. NO. CII. — OCTOBER 1851. Q 



