Even as the Mont Cenis shows a transition period, so the Arlberg may be said to emphasise a triumph of the methods then indicated. So great have been the improvements of the rock-boring machinery, of the power of the blasts, and the speedy ventilation following the explosions, and of the quick transit of materials, that we shall most likely hear no more of sinking numerous shafts along the route. But what of subaqueous tunnels? Violent explosives are hardly suitable for excavation a few feet under a turbid river. What is to be done, when cutting under a full and treacherous stream? CHAPTER IV. UNDER WATER AGAIN. “How to cross the Thames at Blackwall, far east of the Tower Bridge?” That was a problem which the citizens of London had to face in the latter part of the nineteenth century. An immense population dwelt on either side, and some means of easy communication became a pressing necessity. Should it be effected by means of a bridge, fixed or floating, or by means of a tunnel? Finally a tunnel was decided upon, with sloping approaches on either side. Its entire length was to be 6200 feet including the approaches; but herein lay the danger and the difficulty—it was to be driven only seven feet below the bed of the river, and through loose soil and gravel. How then was this perilous task to be accomplished? If the great river burst through Brunel’s fifteen feet, would it not be much more likely to rush through this seven feet of loose soil? But the engineers in charge had an appliance in hand, which was unknown to Brunel—viz., a compressed air chamber, a piece of apparatus which has facilitated several great engineering achievements, besides the Blackwall Tunnel. When the excavation of the tunnel was commenced, a stout apartment was formed at the end of the cutting, into which air was pumped until it exerted a pressure of some thirty-five pounds to a square inch, in addition to its usual weight. 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.