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A TISTORY OF THE BRIDGE. 7
To make assurance fourfold sure, the metal for-this,.as for
every part of the work, has been tested by means of specimen
pieces under the enormous power of the hydraulic press to its
breaking point, a wide margin being always required above
the highest possible strain that it is estimated can ever come
upon it.
All this is plain work. The anchorages are far within-land.
But the great suspension towers to be connected by the cen-
tral span of the bridge must be pushed out to the extreme
wharf line in deep water, for even then the breadth of water
‘to be bridged at one spring is such as no engineer ever attempt-
ed before—nearly 1600 feet—and not only the difficulty but
‘the cost of the work is increased in an enormous ratio by every
foot of added length in a single span. We have therefore be-
fore us here one of the most interesting problems and one of
the most brilliant triumphs of engineering: to build great works
of masonry up from beneath the bed and through the rushing
tides of a.deep arm of the ocean, with all the precision and
-eemented solidity of the dry-land anchorages we have just been
viewing. This part of the work, therefore, was first in order:
‘this achieved, left nothing problematical, whether as to avail-
-ability or cost, in the remainder of the work.
Probably to the end of time thoughtful spectators unversed
in the mysteries of engineering will pause, as they now do,
before these gigantic towers, more wonderful than the Pyra-
mids, with the everlasting sea beating their
angle-irons, of course, be so strong and numerous that nothing
can loosen timber from timber save by tearing each stick into
splinters. Farther, let the back or platform that is to carry
down the great tower in its descent to the bed-rock. be sup-
ported at intervals by six cross partitions of solid timber four
feet thick, with a door in each for communication between the
compartments thus formed. These partitions, like the four
sides, will ultimately rest on the bed-rock, and bear their part
of the monstrous and everlasting load.. Finally, let the whole
cavernous interior be lined with boiler iron, seamed air-tight,
for its perfection as a diving-bell, and for protection against
the danger of fire, which experience in building the first or
Brooklyn tower of this bridge has shown to be imminent at
all times while
working by gas-
light and with
mighty bases, and will perplex themselves in
‘yain to imagine by what means the granite
masonry could have been laid so solid and
true beneath, not forty feet depth of rush-
ing tides alone, but cighty feet below their
surface, on the rock which those tides had
not touched for untold ages.
To explain this mystery in one word, the
submarine portion of the tower was really
built above-water, in the open air, and thence
‘sunk toward its bed as soon as built. But
this is to put a new mystery in place of the
first, for how could such a mass of masonry
be set firmly to a hair’s-breadth in its bed
-against the mighty current, or how could
its bed be excavated to this enormous depth
‘to receive it?
The principle of the diving- bell, ‘sup-
plemented by the air force-pump, or com-
pressor, is the solution of the difficulty.
‘Only the diving-bell must be a’ peculiar
one, made to carry on its back the giant
tower as ‘it dives to the bottom, as it
-delves into the bowels of the earth, and
as it reposes at length and forever on’ the “
rock, It is technically called a caisson (having been first
used in France), from its resemblance to an inverted chest.
Imagine your diving-bell, or caisson, made of an oblong form,
corresponding to the shape and size of its burden, with a mar-
gin of eleven feet excess on all sides. You must, of course,
-also have it built with sufficient durability of material and
-strength of mass both to carry down the masonry entire with-
-out flinching, and to rest under it forever without yielding or
‘decay. It will be best to have the sides of our oblong diving-
bell flare a little, and on the inner side to taper them to a sort
-of edge (well shod with heavy iron), so as to make room for
the laborers within to excavate conveniently to the very ex-
tremity of the dimensions of their diving-bell. To obtain suf-
‘ficient strength and rigidity in the structure for its tremendous
back-load, let its entire top, 102 fect by 172, be built to a thick-
cness of 22 feet of dense Southern pitch-pine in timbers twelve
inches square, laid in solid courses crossing each other, fastened
with powerful through-bolts, and all the joints and seams filled
with pitch. (The bolts and angle-irons of this caisson at, New
York aggregated 250 tons.) Let the sides be eight feet thick
-at their junction with the top, built in the same manner, but
tapered on the inside, as already suggested, down to an iron-
:shod edge only cight inches thick, and let the iron bolts and
blasting
explosives
in com-
presse
air.
Of course there must be means of ingress and egress for
men and materials. There must be a well-hole through the
top, and an iron well Icading to it from the open air above-
water for the men to go in and out. It must be lined with
iron, continuous and air-tight with the lining of the interior,
and must have an air-tight iron door, or rather two successive
doors, with an air-tight chamber between them large enough
for a gang of men to enter, that the outer door may be closed
on them while the inner door is opened to admit them to the
artificial submarine cavern. This chamber is called an air lock,
and its principle is like that of a canal lock, or still more ex-
actly that of a pump. In going out, the men enter the air
lock while its outer door is closed tight, and after the inner
door through which they entered is closed behind them the
outer door may be opened for their egress. Thus the loss of
compressed air by the entrance and exit of a gang of men is
simply what the air lock will contain and no more.
This would be too tedious a process, however, for the re-
TESTING STEEL.