This is generally reckoned at an average of 14·7 pounds to a square inch. We are so used to this pressure that we do not feel it; but let us enter a room where the air has been much more compressed, as in this air-chamber, and serious consequences would be likely to ensue, especially at first.
The human body, however, has a wonderful power of adaptability, and after a time some men get used to the change and can work in the compressed air without injury. But at first it may cause bleeding from the nose and ears, sometimes indeed affecting the hearing more or less seriously, and also causing great pain.
The reason for using this compressed air chamber was to keep out Father Thames. The great pressure of the air resisted the great pressure of the water, and held up the seven feet of soil between.
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.
THE ENTRANCE TO THE AIR-LOCK.
(Men waiting to enter the Compressed Air-Chamber through the Door.)