alue of labor in England and
America, the cost per lineal foot of the iron tubular bridges could
not be less (for the average span of 150 feet) than three hundred
dollars.

5280 feet by $35 is $184,800.00
5280 feet by 300 is $1,584,000.00
The six per cent. interest on the first is $11,088.00
The six per cent. interest on the second is $95,040.00
And the difference is $83,952.00

or nearly enough to rebuild the wooden bridges once in two years;
and ten years is the shortest time that a good wooden bridge should
last.

The reader may wonder why such structures as the bridge over the
Susquehanna at Columbia, which consists of twenty-nine arches, each
two hundred feet span, the whole water-way being a mile long, and
many other bridges spanning large rivers, and having an imposing
appearance, are not referred to in this place. The reason is this:
_large_ bridges are by no means always _great_ bridges; nor do
they require, as some seem to think, skill proportioned to their
length. There are many structures of this kind in America, of twenty,
twenty-five, or thirty spans, where the same mechanical blunders are
repeated over and over again in each span; so that the longer they
are and the more they cost, the worse they are. It does not follow,
because newspapers say, "magnificent bridge," "two million feet of
timber," "eighty or one hundred tons of iron," "cost half a million,"
that there is any merit about either the bridge or its builder; as
one span is, so is the whole; and a bridge fifty feet long, and
costing only a few hundreds, may show more engineering skill than
the largest and most costly viaducts in America. Few bridges require
more knowledge of mechanics and of materials than Mr. Haupt's little
"trussed girders" on the Pennsylvania Central Road,--consisting of a
single piece of timber, trussed with a single rod, under each rail
of the track.

Again, as regards American iron bridges, the same result is
found to a great extent. Thus, Mr. Roebling's Niagara Railroad
Suspension-Bridge cost four hundred thousand dollars, while a
boiler-plate iron bridge upon the tubular system would cost for the
same span about four million dollars, even if it were practicable to
raise a tubular bridge in one piece over Niagara River at the site
of the Suspension Bridge. Strength and durability, _with the utmost
economy_, seem to have been attained by Mr. Wendel Bollman,
superintendent of the road-department of the Balt