narrowed to a comparison between the present arch and a central
independent girder of the same span, including the piers on which it
rests. The small side spans could obviously be left out in each case. The
comparison was made with a view not only to arrive at a decision in this
particular case, but also of answering the question of the economy of the
arch more generally. The following table contains the weights of
geometrically similar structures of three different spans, of which the
second is the one here described. The so-called theoretical weight is that
which the structure would have if no part required stiffening, leaving out
also all connections and all wind bracing. The moving load is taken at one
ton per foot lineal, and the strain on the iron at an average of four tons
per square inch. The proportion of the girder is taken at 1 in 8.

--------------+-----------------------+------------------------+
| Theoretical Weight. | Total Weight. |
Span in Feet. +-----------------------+------------------------|
| Arch. | Girder. | Arch. | Girder. |
--------------+---------+-------------+------------+-----------|
100 | 0.0724 | 0.1663 | 0.1866 | 0.2443 |
220 | 0.1659 | 0.4109 | 0.4476 | 0.7462 |
300 | 0.2414 | 0.6445 | 0.6464 | 1.2588 |
--------------+---------+-------------+------------+-----------+
|<------------Tons per foot lineal.------------->|

It can be seen from these results that the economical advantage of the
arch increases with the span. In small arches this advantage would not be
large enough to counterbalance the greater cost of manufacture; but in the
arch of 220 ft. span the advantage is already very marked. If the table
were continued, it would show that the girder, even if the platform were
artificially widened, would become impossible at a point where the arch