s. Experience
has also led the best iron bridge-builders of America to quite generally
employ girders with parallel top and bottom members, vertical posts
(except at the ends, where they are made inclined toward the centre of
the span), and tie-rods inclined at nearly forty-five degrees. This
form takes the least material for the required strength.

[Illustration: SACO BRIDGE.]

The safety of a bridge depends quite as much upon the design and
proportions of its details and connections as upon its general shape.
The strain which will compress or extend the ties, chords and other
parts can be calculated with mathematical exactness. But the strains
coming upon the connections are very often indeterminate, and no
mathematical formula has yet been found for them. They are like the
strains which come upon the wheels, axles and moving parts of
carriages, cars and machinery. Yet experience and judgment have led the
best builders to a singular uniformity in their treatment of these
parts. Each bridge has been an experiment, the lessons of which have
been studied and turned to the best effect.

[Illustration: PHOENIX WORKS.]

There is no doubt that iron bridges can be made perfectly safe. Their
margin is greater than that of the boiler, the axles or the rail. To
make them safe, European governments depend upon rigid rules, and
careful inspection to see that they are carried out. In this country
government inspection is not relied on with such certainty, and the
spirit of our institutions leads us to depend more upon the action of
self-interest and the inherent trustworthiness of mankind when indulged
with freedom of action. Though at times this confidence may seem vain,
and "rings" in industrial pursuits, as in politics, appear to corrupt
the honesty which forms the very foundation of freedom, yet their
influence is but temporary, and as soon as the best public sentiment
becomes convinced of the need for their removal their influence is
destroyed. Such evils are necessary incidents of our transitional
movement toward an industrial, social and political organization in
which the best intelligence and the most trustworthy honesty shall
control these interests for the best advantage of society at large. In
the mean time, the best security for the safety of iron bridges is to be
found in the self-interest of the railway corporations, who certainly do
not desire to waste their money or to render themselves liable to
damages from