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WITH THE CAISSON CREW 547
"RUNNING A LINE THROUGH AN AIR LOCK"
When the tunnel shield is driven ahead, foot by foot, by the score of pneumatic hammers, each delivering
blows of 340,000 pounds weight, constant engineering survey work is necessary to keep the shield in line.
For the safety of the engineers and the other workers in the pressure area, flying gang-ways, safety
screens and emergency locks are located high in all tunnels, so escape will not be cut off in case of a
"blow" or flooding. Medical locks also are provided with rest rooms, bathing facilities, and hot coffee.
The time spent "in the air" depends on the pressure. The human system is so constituted that it ex­
periences no ill effects or discomfort, except for pressure on ear drums, from a quick increase of pressure,
hut a quick reduction in the same may result in great physical discomfort with serious after-effects,
sometimes fatal. The prevention of compressed-air sickness, known as the "bends", has reached a state
whi-ie the illness is less frequent than formerly.