Note 4 — THE BUILDING OF A RAILWAY.

The amount of permanent wood and iron truss bridges, and of temporary wooden trestles on the Chicago, Milwaukee, and St. Paul is as follows:

Truss bridges,700spans, average93feet,12⁴/₅ miles.

Trestle "7,196" "77"103¹/₁₀"

—————

Total,7,896115⁹/₁₀"

The approximate total number of bridges in the United States was in 1888:

Iron and wood truss bridges,61,562spans,1,086miles.

Wooden trestles,147,1872,127"

–———–——

Total,208,7493,213"

Probably three-fourths of the truss bridges are now of iron or steel, and may be considered perfectly safe so long as the trains remain upon the rails and do not strike the side trusses. The wooden trestles are a constant source of danger from decay or burning or from derailed trains, and should be replaced by permanent structures as fast as time and money will allow.