Levels are described by their names, though they are not always in a strictly horizontal position. Sometimes they dip at considerable angles, owing to the formation of the rock, or the position of the substances to be mined. They are made of various heights, though generally of not more than six feet. The materials used for lining the shaft—that is, brick, timber, or stone—are likewise employed in the levels, and the modes of strengthening in both cases are very nearly the same. Where the work is intended to last more than six or eight years, it is generally set up with stone, and not timbered. Where it is intended to last a long time, and especially if the rock through which it runs is of a yielding nature, it is strongly arched with masonry. Sometimes it is necessary to make an arch below as well as above, for the reason that the flooring of the mine is apt to swell up in consequence of the pressure from below. Masonry used in levels is very much like ordinary masonry, and requires no especial description. [Sidenote: TIMBERING LEVELS.] For timbering levels there are three timbers-two uprights and a head-piece. Sometimes there is a fourth piece, placed at the bottom, known as the sleeper, or sill. This is used, however, only when the flooring is soft, or consists of a substance that is apt to bulge up. The pressure from above, as well as the lateral pressure, frequently bends and breaks the timbers. This bending and breaking of the timbers, occasioned by the settling of the earth, are rarely sudden in their occurrence. At first there will be observed a slight bending of the timbers; the next day the bending will be seen to be greater; and sometimes a month, or even six months, may pass before the timbers are sufficiently curved to break. Frequently levels that have been made five or six feet in height will, in the course of a few months, be reduced to a height of not more than three feet. The timbers will be bent around in all directions, and it requires considerable nerve to pass between them. Where the mines are moist, the timbers soon become covered with fungus, and a vegetable peculiar to the interior of the mine makes its appearance. Sometimes it is not unlike light cottony material; occasionally it is snow-white, and again like tanned leather, or of a bright yellow color. The timber, when rotting, has an odor like that of creosote, and is familiar to everybody who has passed any time in deep mines. Rats abound in mines, and are frequently very numerous. They make themselves at home, and are as comfortable as possible. While the miners are at their dinners, they frequently play around them, and appear on friendly relations with them. Occasionally, they become so hungry that whenever a candle is placed in the wall, and the miners back is turned, the rats will rush forward and seize the prize at the risk of being burned. [Sidenote: ACCIDENT CAUSED BY RATS.]