Los Angeles herald., 1907-01-13

tnc world, and it is curious to trace their
gradual rise from the trunk of tree thrown
across brook to the great mass of iron or
masonry OF the present day
The bridge that Horatius kept so we'll was
putt OF piles in fours driven into the bed of
the l1ber, with planks laid from one group
W , 1he other-a very carly pattern.
nose OF the Chinese were suspension
bridges ot bamboos-a device that would seem
20 80 nave come in later; while there is across
ene EutleJ, in India, bridge said to be quite
Us oid as ANGUS Martus, which is actually u
cantliever, with 80 feet imbedded in the ma
smiy' oo feet projecting and centre beam
ming up the gap.
ce ne most curious bridge in England is at
Sioyiahd' IN Lincolnshire. II is Gothic
bridge, built in 1880, in which three road
ways meet in the middle. Where it stands
the We[and River divides into two branches,
ana the bridge has thus to provide for three
water courses as well as three roadways.
:. me object of all bridge builders nowadays
IS 10 give level road, and for that purpose
IRON and wood offer facilities which stone and
Drick cannot claim. Consequently, all our
modern bridges are of iron or steel which is
we coinmonest form of iron now used and
the principle of construction is chosen with
resaru TO t6e work the bridge will have to do.
l., some bridges are suspension" some "tubu-
lar some "bowstring," some "lattice girder,"
some "cantever." The bridge over the Firth
OF OF Forth. Scotland, at present the longest and
nhesI I modern bridge in the world, is on the
cahtlever principles that is to saw it is "a
structure overhanging from fixed base"
Inc river at Queensferry is divided by the
lsIand OF lnchgarvie into two channels, who's.
uepth OF two hundred feet was thought to
preclude the building of intermediate piers.
On the three main piers are three double lat
tlce-work cantilevers like scale-beams, each A
quarter OF mile in length, reaching toward
eaGh other and joined at their ends by two
pprdiRary pra nary glrders $50 feet long. The bridge
60 feet, being the shore ends of the outsr
99aahtievers ca and fifteen spans each of 16S feet.
me Total length of the road is a mile and a
iha1, OF which a mile is taken up by the canti-
levers. Ihe headway under the centre Of
highest part of the bridge is 361 feet above
the same level
Whenever possible, a bridge is built on
soild s, rock If no rock is available. "cof!er-
Ihe coiier-dam is merely a water-tight tank.
sunk full OF water onto the bed of the stream
wnen the water is pumped out and the foun,
dat.on n, and masonry built within it.
the coffer-dam is impossible recourse is
mad to piles A pile is a beam of wood or
IRON driven into the ground by series of
heavy blows, generally given by "monkcy."
d wnIcn N monkey 10 is merely mass of iron drop
were hammer, head, the blows being of mod
crate force, 1T having been found that many
hight blows do the work more etfectually than
few heavy ones.
In sand the piles are now generally sunk bv
means OF the water-jet. An ordinary gas-pipe
IS fixed alongside the pile and sunk with It.
A powerful pump. is fixed to the upper end
OF the pipe, and the water, forced downward
clears the sand away below the pointed end Of
the pile, and down go the pile and the iet
together VIZ 'e'
The jet must never stop until the pile is in
position, for the water rising upward around
the pie renders It's descent as easy as if it
were creased. AS soon as the PUMPING ceases
the sand settles around the pile and firmly
secures it. ''
Sometimes the water is driven down the
hollow pile and squirted out at the point so
as to, clear, the sand away before it. Occa-
sionally piles are fitted with screws and
screwed down into the sand
But of late the foundations of our laroer
bridges have been put in by means of fhe
"caisson'' ''-famiiarly pronounced cassoon-
which Is merely an adaptation of the pr;no;
pie of the diving-bel p'
The caisson is room made of wood. with
the bottom edges of the walls shod with iron
1t is filled with air, and slowly sinks as the
earth IS dug into within it, the pressure of
the air increasing as it descends, and beinr
kept sufficient to prevent the water from make
ing its way in.
The men get into the caisson down tube
which has in it an "air lock-" the air lock
being simply chamber with valves at tbe
top and bottom, on the same principle as a
water-lock IN which boats are lifted or low
cred to different levels on our canals. '""
The lower valve is kept closed by the pres.
sure, of the air pumped into the caisson be-
meath. When the lock is entered the unner
valve is shut, and the small valve connected
with the caisson is opened, 80 that the nres.
sure in the lock soon becomes the same as