Powerful engines were maintained at work to provide for the pressure of the air, and the chamber in which the compressed air was kept was entered and left by the workmen through an “air-lock”—that is, a small ante-chamber having two doors, one leading to the compressed air and the other to the ordinary atmosphere, and neither being opened at the same time. The men, then, worked in this compressed air chamber, which prevented irruptions of the river. But the method of excavation was also another safeguard, both against irruptions of water and of earth. In essence, it was much the same as that pursued in boring the tunnel for the South London Electric Railway; that, however, was through thick clay and about 10½ feet in diameter, and this was 27 feet across, and through loose and stony stuff. The shield, instead of containing as in Brunel’s time a number of cells, consisted of an immense iron cylinder, weighing some 250 tons; closed in front, but having a door in the closed part; the rim of the cylinder round this part having a sharp edge for cutting into the soil. [Illustration: THE ENTRANCE TO THE AIR-LOCK. (_Men waiting to enter the Compressed Air-Chamber through the Door._)] The door being opened, the men found themselves face to face with the earth to be excavated. They cut away as well as they could, perhaps about 2½ feet deep, throwing the earth into trucks in the compressed air chamber; these trucks would be afterwards hauled away through the air-lock by electricity, and the huge iron cylinder would be pushed forward by means of hydraulic power. Twenty-eight hydraulic “jacks” were employed, and they forced forward the 250 ton cylinder with its cutting edge, when the men would resume working through the door as before. Behind them, the hole of the tunnel thus cut out was being lined. First, it was built round with iron plates a couple of inches thick. This plating was fixed in segments, and formed a huge pipe a little smaller than the actual hollow in the earth. Through holes in the immense piping, liquid cement was forced, thus plugging up the space entirely between the earth and the iron, and forming an outer ring of cement. Within, the tunnel was completed by a facing of glazed tiles, placed on a thickness of 14 inches of concrete. A road-way was laid 16 feet wide, flanked by footpaths of 3 feet, 2 inches, on either side. The subway is lighted by electricity, and staircases on the banks lead down to it for foot passengers. The stairways give entrance to the tunnel not far from the river, and much nearer than the commencement of the carriage-way approaches.