The flexible tubes conveying the compressed air and the water are detached from the machines, and placed in security in the covered canal. The perforating-machine, mounted on its frame or truck, is drawn back on the tramway behind two massive folding-doors of wood. Miners then advance and charge the holes in the centre with powder, and adjust the matches; fire is given, and the miners retire behind the folding-doors, which are closed. The explosion opens a breach in the centre part of the front of attack. Powerful jets of compressed air are now injected, to clear off the smoke formed by the powder. As soon as the gallery is clear, the other holes in the perimeter are charged and fired, and more air is injected. Then comes the third operation. Gangs of workmen advance and clear away the debris and blocks of stone detached by the explosion of the mine, in little wagons running on a pair of rails placed by the side of the main tramway. This done, the main line is {70} prolonged to the requisite distance, and the perforating engine is again brought forward for a fresh attack. Thus, we have three distinct operations--first, the mechanical perforation of the holes; secondly, the charging and explosion of the mine; and thirdly, the clearing away of the debris. By careful registers kept since the commencement of the work, it is found that the mean duration of each successive operation is as follows: for the perforation of the holes, seven hours thirty-nine minutes; for the charging and explosion of the mine, three hours twenty-nine minutes; for the clearing away of the debris, two hours thirty-three minutes; or, in all, nearly fourteen hours. Occasionally, however, the three operations may be completed in ten hours, all depending upon the hardness of the rock. It has been found practically more expeditious to make two series of operations in twenty-four hours.