The descent is effected on foot through a vertical cylindrical shaft used for that purpose only, and divided at intervals by platforms which communicate with one another by good broad wooden staircases. The visitor is provided with a lighted candle attached to the end of a stick, which serves at the same time as an excellent test of the purity or impurity of the air in the mine, for the lower he descends, the more frequently he will find his light to be extinguished by carbonic acid gas, arising chiefly from the exhalations of the convicts. There are no inflammable gases in the mine, and the men work with naked lights. As he descends ladder or staircase after staircase, the visitor becomes conscious of the presence of human beings in the mine, for strange unearthly sounds greet his ear more and more plainly as he approaches the long gallery which traverses the mine at about 110 feet below the surface; and this effect is rendered still more weird through the surrounding darkness, relieved only by the faint light of his candle and those of his companions. From moment to moment he hears hollow echoes of the human voice uttered in snatches and accompanied by a continuous clanking of chains, which makes his blood creep until he has become to some extent accustomed to the sound. The shaft through which he is descending is cut and rounded with great precision, first through a mixture of clay and rock-salt, and then in the solid rock-salt itself. To render it impervious to water he will find the wall here and there lined with buffalo hides.[74] Arrived at the horizontal gallery the visitor passes along it until he comes to a platform guarded by a fence or railing, and then he finds himself near the roof of an enormous cave which is probably unlike anything to be seen elsewhere.