The rubbish and running sands are removed in buckets by hand, or by means of a rope passing through a pulley. Trapdoors communicate from one stage to the other, by means of which the buckets are removed without any serious loss of the compressed air. Shafts may be sunk through quicksands in this way to a depth of eighty or one hundred feet without difficulty. The laborers who pass their time in the compressed air work as easily as in the open atmosphere. Some of them, however, cannot remain there long, especially if they have the drum of the ear very delicate, or are in the habit of drinking to excess. The pressure of air in the chambers rarely exceeds three or four atmospheres. [Sidenote: AN INGENIOUS APPARATUS.] This apparatus is frequently used for laying the foundation of bridges in the beds of rivers, where there are deep quicksands. The famous bridge of Kehl, near Strasbourg, was constructed in this way, and the engineers say that without some such apparatus the construction of the bridge would have been impossible. If a shaft has been sunk and properly supported,—that is to say, timbered or walled,—it is generally divided into compartments. The shafts are generally from fifteen to twenty feet in diameter, and consequently there is plenty of space for dividing them. One of the compartments will serve for the tubs, cages, or buckets, in which the coal is raised. Another is for pumps to draw off the water, and sometimes where the miners go up and down by ladders a compartment is made especially for them. [Sidenote: LEGAL NECESSITIES.] In all cases one compartment in the shaft serves as an air-way or chimney, whether the draft is free or not. In some countries the law requires that there shall be more than one shaft, or opening, to every mine, while in other countries no such law exists. Many of the owners of mines are abandoning the single shaft system, and gradually supplying their mines with more than one entrance. Many terrible accidents, accompanied by a great loss of life, might have been avoided had the mines been constructed with more than one entrance or shaft. A striking example of this is in the terrible calamity at Avondale, a few years ago. The most approved arrangement of shafts for a large mine where there is explosive gas, and where water is to be pumped, is to sink one shaft for the pumps, another for raising coals, and a third for ventilation. At the bottom of the third one a large furnace is always kept burning. In some of the mines there may be half a dozen shafts. Those through which the coal is drawn are called the winding pits, those where the pumps are fitted are called pumping pits, those where the men go up and down, are called labor shafts, and those for the passage of air are known as air shafts.