THE KANSAS CITY BRIDGE. 79

remaining four were respectively examples of the Post, the double and the single
triangular, and the Fisk suspension trusses. On the 30th of October the con-
tract was let to the Keystone Bridge Company, of Pittsburg, Pa.; the fixed
spans were to be built according to the plans supplied by the chief engineer,
the iron in them being paid for by the pound, and the timber by the foot ;
the draw was to be built, according to the contractors’ design, for a fixed sum ;
subject, however, to such alterations as might be suggested by the chief
engineer, the Company to have the benefit of any saving which might result
from such changes, and to pay any extra cost which they might involve.
Under this provision certain changes were suggested in the depth of the truss
and arrangement of panels, which resulted in a material reduction of the cost.
By a subsequent arrangement a pony truss of wrought-iron, made by the con-
tractors from their own designs, was built, in place of the composite structure
proposed by the engineer for the shore span of 66 feet.

The general design of the fixed spans is that of a double triangular truss
or trellis girder, in which the top chord, posts, and braces are of wood, and the
other members of wrought-iron, cast-iron being used in the details and con-
nections. This combination, which has been used as yet only to a limited
extent, is believed to overcome the most objectionable features of a wooden
bridge, avoiding the wasteful connections which accompany the use of wood in
tension, and disposing of the bulk of the perishable material in places where it
can easily be protected ; besides this, the character of the butt-joint connections,
used to take compression, is such, that worn out parts can be removed and
replaced by others without disturbing the remaining parts of the structure; it
is also possible to replace the wooden parts by iron, and thus gradually convert
the bridge into an iron structure without the expense of false-works or the
intermission of traffic, The braces, which are always open to the air on all
sides, are exposed to moisture only during the actual prevalence of a storm,
and would therefore be well protected by a thorough coating of paint. The
top chord can be covered in, and thereby thoroughly protected from the weather,
without perceptibly increasing the wind surface of the bridge. The only danger
to which such a bridge can be exposed is that of fire, and if the wood-work be
painted throughout with mineral paint, and a watch kept, which is always