On the Mammoth Cave of Kentucky. B35 
our crossing them in the morning—the rain which had fallen co- 
piously having already affected them. 
was at a loss for a little time to account for these currents of 
air, but the following explanation suggested itself. The mouth 
has formed them. The air which they contain is pure and ex- 
hilarating. 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 proportion of free oxygen disengaged, thus 
enriching this subterranean atmosphere with a larger portion of 
the exhilarating principle. 'The temperature of the cave is uni- 
formly 59° F., 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 1s Im- 
mediately felt by the denser air of the cave, and it flows out in 
obedience to the law of motion in fluids, and the outward cur- 
Tent continues without interruption as long as the outer air 1s pos- 
ere is no exception to the purity of the air in the Mammoth - 
“ited. The waters of the springs and rivers within it are all limpid 
and potable, and the avenues with few exceptions are dry at all 
Seasons. I thought it very remarkable that in all its vast extent 
sv a 
