The Helena independent., 1891-11-03

ches deep. They also gave up the work
tn shovels and pilot and adopted the
bach shield for pushing through the silt.
his shield was hrst used to build the OK
rlmental rapid transit tunnel that still
Ps 800 feet under Broadway from Warren
rect to the present New York postoffice.
nat was in 1869-7O, and later it built tun
lis under the. Thames, and still later the
Clair tunnel. The shield that now closes
Q end of the completed portion OF the
udson river tunnel consists of cylinder
n feet long and twenty feet in diameter.
one foot eight inches larger around than
9 carbon tunnel itself A few feet of its
ner edge always overlaps the plates form
tho facing of the tunnel. 10 as to allow
opportunity for the caving in of the soil.
bis great shield weighs eighty tons. 1t is
aced around its inner edge, and when
Ishing braced against the last plates Of
9 tunnels facing. Each jack is capable
exerting driving force of fifty tons.
While the shield WAS to render possible q
duction of the air pressure in the tunnel
low what had been required for safety
lder the old system, its introduction there
fragments was far from having that ef-
ct. To build it in it's correct position q
amber much higher and wider than the
nnel had to be excavated. The soft silt
bills of this chamber were sustained partly
timber. but mainly by the resistance Of
impressed air. For several weekD the
en- worked in putting this shield together
Ader pressure Of forty-three pounds.
hey also were subjocted to an exceedingly
oh temperature on account of the heat
ON the riveting furnaces. which were
pt burning in the tunnel night and day.
II was during the building of the shield
hen the ambulance look, as it is known at
tunnel, came into existence. So many
on had tho ''bends." disease to which
oso working in compressed air are sub
OF, that something had to be done. Three
en- died at the entrance OF the tunnel
fore medical aid could be summoned,
ID many went to the hospital suffering
mo the ambulance lock has done good
rvice. II is in shed adjoining the engine
puse and is nothing more than hollow
Fought iron boiler, twenty feet long and
feet in diameter, divided into
10 compartments by an iron par
tion. The low doors entering from
. outside and to the second cham
r of the huge cylinder are of heavy
On and fit it tight by means OF rubber
" the result of being in too much Of q
hrry in leaving the tunnel. After having
ben in the comp sssed air for several hours
in necessary to spend some time in the
cks that form the gatsways between tho
dersnt sections of the tunnel, where the
essure IL less than in the compartment
. man has left, but its still higher than
10 normal atmospheric pressure. Men
lo neglect this preoaution are often. on
inning out Of the tunnel, seized with severe
amps. In seversoases they lese all power
/ motion or speech. Now when person
10 the bends at the Hudson river tunnel
il hustled iato the further COM
krtment of tho .embulante lock-. The
bors ars soloseti and''a pressure of air 10
rned on. II an attendent has gone with
o sufferers into the lock he can Withdraw
ter tho compressed air has entered by
ssinH into the outer compartment and
stenine the door behind him: he il then
40 to leave the lock by the other door.
hs contracted joints of the victim of the
bndx generally relax soon after the gauge
tho front Of the ambulance lock shows
at there IL thirty pound pressure inside,
hd then the pi sssurs Ia gradually reduced.
~ have been taken into this big cylinder