Omaha daily bee., 1897-06-27
M and perfect goods VIII be SOIl al Ila|1 me pIIC

fined space where it did it's duty from day
to day and from night to night.
THE INFLOW OF WATER.
All so far seems easy and simple, but now
comes to our minds conditions that make
success well nigh impossible; for 1t occurs
to us naturally to ask how the water which
filled the soil was kept out of the way so that
the men could work safely under a river so
wide and deep. We know that water presses
equally in all directions. and that a volume
of water one-eighth of an inch wide Will
exert just as much pressure as a body of
water an ocean wide. That is what is called
the ''hydrostatic paradox" simply because
1T does not seem reasonable. We know also
that water is well nigh incompressible
and we know that air is compressible.
If we turn a tumbler bottom side up and put
IL into a tub of water and push 1t down under
the surface, we notice that there is still air
in the tumbler If now we put the tumbler
down to the bottom of the tub we see the air
is still there, but occupying less space than
before. The knowledge Of this fact led to
the use Of the diving bell, a great iron box,
say eight feet wide, sixteen feet long, and
ten feet deep, turned bottom up like a
tumbler, with thick circular glass windows
set into the top and lowered down into the
bottom of a river or sea so that men could
stand in the top Of it inside and work. Then
1t was discovered that air, compressed by
means of pumps, could be forced into the top
VI thlS DCll uilf III hoANA ' va " NfS
very bottom of it, so that men could stand
dry shod on the rock or ground and work
as on shore. This principle was applied TO
the Blackwell tunnel. A wall or bulkhead of
brick, twelve feet thick, was built across
the tunnel, then air composers, driven by
engines of about l,OO0-horse power, forced
the air down through pipes into the working
section of the tunnel, which lay between this
wall and the shield at the other end; and
the men worked there in the dry. The total
air pressure thus formed against the bulk
head was 900 tons. The plant, that is the
pumps of various kinds, air and water, were
placed above ground near a shaft already
described, and the air and water under
pressure was led by pipes down the shaft
and through the bulkhead to the point in
the working chamber, where the power OF
various kinds was needed.
WORKMEN UNDER AIR PRESSURE.
The pressure of the water is in proportion
to its depth, and is about two-ffths OF a
pound to every foot of depth. Eighty feet
of depth would therefore make a pressure
of thirty-two pounds more than the ordinary
pressure under which we uOually live at
sea level, which Is fifteen pounds. In other
words, the men had to work in an air pres
sure of more than twice that of the air
above. By going slowly from the outer air
into the condensed air one can live and
work; and so what is called an air lock
was built into the brick wall. The men
stepped into 1t and shut the outer door. A
valve let the compressed air slowly into
the lock from the inner working chamber
This lock Is a horizontal steel tube fifteen
feet long and six feet in diameter. The air,
in rushing in, sounds Just like that es-
caDing from the cylinder of an aiTbreak
under a railroad passenger car, When the
pressure in the lock is the same as that
in the working chamber of the TUNE the
inner door is opened, and the men go out
OF the lock into the chamber.
AnuF thig aNH 70 nrl n ~ nrnGgnnn that
oYerbalances the water pressure and drives
the water out, also endeayore by every pos
sible means to escape from its confinement,
and such was the loose nature of the porous
material in the bed of the river that there
was a constant escape of the air into the
Thames above. 1t required the dumping Of
a great clay blanket several feet thick
over the whole of this part of the river
bed, and even then the air in the tunnel
blew out through it twice, throwing up
water at one time twenty-five feet high
over an area Of nifty feet in diameter. so
that the boatmen above thought a great
torpedo had exploded.
Another serious condition had to be wisely
met. We have stated the facts about the
water pressure, that it increases twO-ffths
of a pound with every foot OF increased
depth. The tunnel was over twenty-seyen
feet in diameter. This would give about
twelve pounds per square inch greater pres
sure at the bottom than at the top Of the
tunnel, Now, If sufficient air pressure were
used TO drive the water back out Of the bot
/ tom, It would be stronger than was neces-
sary at the top of the tunnel by twelve