St. Tammany farmer. [volume] (Covington, La.) 1874-current, 1902-10-11
WHEN BRIDGES BREATHE.
Some Interesting Facts About the
Lungs" Of the Worlds Great
Iron Structures.

Though to speak of bridges breath
ng appears passing strange, neverthe-
less, inanimate structures are never
still for moment during the live
long day, says London Answers.
The Britannia bridge, for instance,
which is 400 feet long under normal
circumstances, is from half to three
inches longer at three p. m. than it is
IL hours later, according to the
amount of sunshine it is subjected to,
which draws it sometimes an inch up
ward if the heat strikes the top of the
tube, or to one side when the sun in
creases its temperature laterally,
though the heaviest train will only
bend it half an inch at any time. In
like manner intense cold will cause a
bridge to shrink more appreciably.
To show how intense the cold must
have been, we have only to point out
that Southwark bridge over the
Thames, which, however, is only about
one-ninth the length of Brooklyn
bridge, only rose in the center to the
amount of 11a inches for 50 degrees
rise in temperature. To allow for ex-
pansion or contraction, the builder of
the Clifton suspension bridge provided
the ends of the roadway with hinged
flaps eight feet long, which allow of
perfect freedom of movement; and the
makers of the tower bridge, which is
about 211a inches longer in the summer
than in winter made similar allow
ance.
After the choir of Bristol cathedral
was covered with sheet lead in 1851 it
was discovered two years later that,
notwithstanding the fact that the
length of the covering was 60 feet,
and it's depth 10 Is feet, it had
crawled down bodily for 10 inches,
drawing out nails from the raft
ers in it's course, which had been
driven in to arrest its progress.
The explanation of the movement lies
in the fact that the lead naturally ex-
banded more freely to the suns heat,
and that when it contracted at night it
drew its upper edge after it, in prefer
ence to climbing up to it.
The Hffel tower, like the Britannia
bridge, is ever on the move, either up
ward or downward, according to the
temperature, the summit of the tower,
so it is said, entailing an extra climb of
five inches, when the temperature is
high than in the cool of the day, while
in winter it is eight inches shorter than
at midsummer. Even the white marble
obelisk dedicated to Washington,
which, erected at cost of 100000,
rears it's head 555 feet in height on the
bank of the Potomac, is not proof
against the power of the sun, and is
said to increase its height by two
inches and to bend slightly on hot
day.
With regard to the latter movement,
a copper wire ia feet long, carrying a
plummet suspended in a vessel of wa
ter, renders perceptible the slight
bend of the shaft caused by the rays
of the sun pouring on one side only,
though the inclination Of the apex of
the monument amounts to but few
hundredths of an inch toward the
north each day atnoon throughout the
summer.
The railway line is very powerful
respirator. Probably every Answerite
has noticed the gap between each rail.
In winter this chink will be over a
quarter of an inch in width, but in
summer it will be quite closed up.
During one of the frequent civil wars
incidental to South America one of
the belligerents conceived unique
method of train-wrecking. During
the night he drove steel wedges tightly
into these gaps for considerable dis