Far superior to this mode of ascending or descending are the safety-cages introduced of late years, which have guides the whole length of the shaft, and bonnets or roofs to protect the heads of the men within. They are made with several stages, in which either the tubs or waggons can be placed, or where the miners can stand or sit. If a rope breaks, a spring placed above the cage and kept taut by the tension of the rope, is set free, and acts upon a double clutch made of the best tempered steel. This catch or wedge falls between the wooden guide and a part of the cage, and brings the latter immediately to a stand-still. By this means numberless accidents have been prevented. The man-engines which have been described are dangerous for novices, for should a person stop at the wrong time, he may be hurled to the bottom, or crushed at the return stroke. One of the most frequent accidents to which miners are exposed arises from an outbreak of fire-damp. To avoid this, various safety-lamps have been invented. The most celebrated is that known as Sir Humphrey Davy's lamp. The flame is enclosed in a fine wire gauze, through which, under ordinary circumstances, the gas cannot penetrate. There are other lamps in use constructed on the same principle, but superior in some respects. Too often, however, the miners open them at some fatal moment, or enter the mine, against orders, with naked candles. Still, by means of these lamps, when properly employed, many accidents have been prevented. Another invention exists by which a person can enter in the midst of impure air. The apparatus was devised by Monsieur Kouquayrol, a French engineer. It consists of a reservoir made of sheet iron, into which the air is forced, and, by an ingeniously contrived pump, is secured like a knapsack to a man's back, and the air is conveyed by means of a tube to the mouth of a nose, and thus into the lungs at the ordinary pressure, while a small external valve allows of the escape of the air after it has been respired.