ke sure of
good iron in the beginning, and that we can also be sure that it does
not decay; while, however good our timber may be in the beginning, we
never can be entirely sure of its condition afterwards. There are
wooden bridges now standing in this country, all the way from sixty
to eighty years old, which are apparently as good as ever; while
there are others, not ten years old, which are so rotten as to be
unfit for use. It will not do to assume, that, because no defects are
very evident in a wooden bridge, therefore it has none. When a wooden
bridge, originally made of only fair material, has been in use under
railroad trains for twenty-five or thirty years, and in a position
where timber would naturally decay, we are bound to suspect that
bridge. To assume such a bridge to be all right until we can prove it
to be all wrong, is not safe. To assume a bridge to be all wrong
until we can prove it to be all right, is a safe method, though not a
popular one. Any person who has had occasion to remove old wooden
bridges, will recall how often they look very much worse than was
anticipated.

There is one defect in railway bridges which has often led to the
most fearful disasters, and which, without the slightest question,
can be almost entirely, if not entirely, removed, and at a moderate
cost. At least half the most disastrous failures of railroad bridges
in the United States have been due to a defective system of
flooring. With a very large proportion of our bridges, the failure
of a rail, the breaking of an axle, or any thing which shall throw
the train from the track, is almost sure to be followed by the
breaking down of the bridge. The cross-ties are in many cases very
short, and the floor is proportioned for a train _on_ and not _off_
the rails. When an engine on such a floor leaves the track, it
plunges off the ends of the cross-ties into the open space between
the stringers and the chords, and generally wrecks the bridge. To
prevent this, the cross-ties should be long and well supported, and
placed so close that a derailed engine cannot cut through them. The
track should also be provided with guard-timbers well fastened, and
the width between the trusses should be so great that the wheels of a
derailed train will be stopped by the guard-rail before the side of
the widest car can strike the truss.

The importance of a substantial floor system has been very fully
recognized by the railroad commissioners of Massa