What are these wooden pipes along the floor? They were laid there by the saltpeter miners to convey the water from the cascade at the entrance down to the leaching vats that are now pointed out to us in the Rotunda. The ruts of old ox-carts are visible in which the “peter-dirt” was carried to the vats from the open avenues, while sacks were used for those more remote. The solution was pumped out to open-air boilers, run through ash-hoppers, cooled in crystallization troughs, and packed for transportation to the seaboard, mostly by mules. Thus did patriotic Kentucky supply the government with one of the ingredients of gunpowder, at a time when foreign sources were cut off. The yield of nitrate was four pounds to the bushel of soil, and the vast heaps of lixiviated earth seem to warrant the boast that Mammoth Cave alone “could supply the whole population of the globe with saltpeter.” The lofty arch of the Rotunda is directly under the Hotel; and it would be possible, by means of a shaft, to supply every room with the sweetest and purest air, transforming it into a “lime-air” sanitarium, as has actually been done in some other localities. By a series of temperature observations with verified thermometers, the writer has proved that the uniform temperature of the Cave is 54° Fahr., winter and summer; and the air is chemically and optically pure. Lighted by magnesium fire the grandeur of this first of many halls is made visible, as are also the openings of two broad avenues, one of them being the Main Cave and the other the entrance to Audubon Avenue and Little Bat Avenue. Here in winter assemble myriads of bats from all the region around, clustering in nooks and crevices for their long sleep of hibernation. [Illustration: Entrance to Mammoth Cave] Leaving Audubon Avenue to be described in Route II, we enter from it, at a point some five hundred feet from the Rotunda, and by a low arch, the winding way known as Little Bat Avenue, chiefly remarkable as leading to the Crevice Pit, which is immediately over the Ruins of Karnak. The story is told by the late Dr. R. M. Bird that a former owner, Mr. Wilkins, let a lamp down the pit by a rope that caught fire, with the loss of the lamp. A reward of two dollars was offered for its recovery. A little darkey agreed to be let down, as a sort of living plummet, to sound the depth of the chasm. He told such a tale as to the magnificent temple underneath, with its tall columns and splendid adornments, that nobody believed him. Thirty years later the lost lamp was found by old Matt, the guide, who gave it to me. The Main Cave, or Grand Gallery, or as we like to style it the “Broadway” of this subterranean metropolis, extends from the Rotunda to the Cataracts, and must be traversed to reach any other part of the cavern. In this first route only about eight hundred yards of it are shown.