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.