The Manchester journal. [volume] (Manchester, Vt.) 1861-current, 1870-11-29
gBrsr Lotfs RAD

iubu' ar. cast steel arch bridge sappar
ted by the abuin eats snd tWu peers ths
iauprare 5t5 feet apart sud iGI feet
Ieach from its neacst abutmcUt making
thrce span oiahoat aoa feet each Is
greatest span Is tbe samc as that Ut the
/ lkuilleuburr bridge. owe the l.eek, an
arm Of the Rhine, Ia Holland Thomas
Telloid's suspension bridge aero the
Mcna Straits, 10 the nortl western part
af Males. has a span ot $40 ieet lae
Victoria tubular iron bridge Of Montreal,
exceeds ihi. greatly ia length, being G,699
II Ia miles,. but il rests upon q piers,
and is spans are ouly 255 feet above the
water The Last River Bridge will span
+ coo feet, At height midway ofl30 feet
But the novel method of The coustruc-
ton Of this bridge in some particulars
renders it especially worthy OF note. The
pier. are sank in the following manners
The masonry Is commenced at the surface
Of the water, upon an inverted eliptical-
shared casson, 80 feet long by 40 wide
the dimensions of tie pier. This is closed
at the top aud open at the bottom, with
rs lower part larger than the upper 20
facilitate It's passage through the sand uf-
ter IL reaches the bed OF the river. IL
would be very much like building the pier
upon the bottom OF an inverted washtub,
OF the same size and shape as the casson.
The caissou is filed with air, like diving
bel, and the mass Of masonry which
constantly accumulates upon il, is borne
up by the confined air, and as the cassou
dccends, the pressure OF the wafer con-
deuscs the air SO that the water rises com
sidcrably within it, just as when an in-
yeried tumbler is pressed down upon a
vessel Of water. TO prevent this and give
greater boyancy 10 the mass, air is forced
Into the caisson through vertical passage
Ia the masonry by powerful steam pump.
The casson with its superstrnciure of ma.
somy must be sunk TO The rock bed OF the
river, because the deposit ofsand above
( which at one pier is ~ feet deep--iu
times of food and freshel, is secured
away TO a great depth, il not TO The rock
itself. When the caisson reaches tin
river bed, the sand within il must be re-
moved. This Is done by current OF
water that is forced down, by tube,
1 through the masonry, into the caisson and
then up again to the surface and as il
takes It's upward course the sand is shovel
cd into il through contriyance for til,
purpose, aud carried to the surface in the
form of muddy water by the ascending
current and poured out into the river
4 Here it causes bank OF sand to accumu-
late which sometimes rises TO the surface
OF the water
Workmen are needed IN this caIsson Of
condensed air, below the bed Of the river,
TO shovel the sand and do other necessary
work. These pass down by means Off
circular stairway IN another vertical pas
sage-there being five in al--lhrongh lh
center OF the pier, and are admitted into
the caisson through an air-lock or cham
Der, with an air-tght door on the uppD
and lower sides. Into this chamber al
ter the men have passed the upper door
the condensed air is gradually admitted
ill it is as dense in the lock as III the Gals
son below. They experience here very pc
culiar sensations, among which are, "A bur
densOmc pressure upon the whole system
aud especially upon the drum Of the ear
and a great increase Of heat in the sys
tenn, because condensed air has smaller
capacity for rheat than in its ordinary state
In passing out, Of course, the order o
proceeding and Of sensations is reversed
rIe w0rkmcn IN the caisson are exposet
to considerable danger from the unusua
atmospheric pressure, which sometime
amounts 10 two or three atmosEheres. -
Several have died from the injuries hef
received. When the caisson has eaehel
The bed rocg, the bed dips" towards tn
Illinois shore, Is lcveled Of with concrete
aud the solid pier rests upon foundation
of limestone rock. Two piers have bee
sunk in this manner and are now aboy
the surface Of the water; the last will b
similarly sunk this fall
The manner oftcstng the steel whc
win form the arches of the supcIslructm
is also very interesting. This is done b
means ofa massive machine which acts b
hydrostaic power. By its use the powc
Of the steel to resist both compression au
, tension is accurately determined. II is
we'll known principle of hydrostatics- tht
a given pressure upon one square iuch {
liquid surface causes equal pressure upo
every iuch Of that surface. THE instr
= mcut is so constructed that the turlaJ
where The power is cxeted is TO the ,u
" face Of the piston where the power il AZ
: Piled as / 10 100: hence the exertiou ofor
pound Of power produces compfessio
I OF tenril force Of l09pounds ANY chang
! 10 the length Of the steel 10 be tested ,eyC
= to Tie 20,0goth part Of an inch il is Sal
= can be detected. Tbs change Is indicate
by a mirror, which reyolges as the pislo
= moves, au1 which reflects light from
4 graduated arc, 70 feet distant, to a leIe
/ cope situated in the arc Through III
3 the observer looks aud records the col
9 tiuued changes Of The steel By the viyic
pressure TO which il Is subjected Ever
s piece Is tested until rs limit Of elaslieil
p i reached, that Is until il has become
= compressed thai II viii not slring ba,
= when the pressure is removed II ma
! be subjected 70 force OF lu9 tons.
Il Won are 4 wise man. van viii trn.