Iron and wood truss bridges, 61,562 spans, 1,086 miles. Wooden trestles, 147,187 2,127 " -------- ------ Total, 208,749 3,213 " 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. [5] See following article on "Feats of Railroad Engineering," page 86. [6] For fuller description of work in a caisson see "Feats of Railway Engineering," page 69. [7] See "Feats of Railway Engineering," page 55. FEATS OF RAILWAY ENGINEERING. BY JOHN BOGART. Development of the Rail--Problems for the Engineer--How Heights are Climbed--The Use of Trestles--Construction on a Mountain Side--Engineering on Rope Ladders--Through the Portals of a Cañon--Feats on the Oroya Railroad, Peru--Nochistongo Cut--Rack Rails for Heavy Grades--Difficulties in Tunnel Construction--Bridge Foundations--Cribs and Pneumatic Caissons--How Men work under Water--The Construction of Stone Arches--Wood and Iron in Bridge-building--Great Suspension Bridges--The Niagara Cantilever and the enormous Forth Bridge--Elevated and Underground Roads--Responsibilities of the Civil Engineer. There are one hundred and fifty thousand miles of railway in the United States: three hundred thousand miles of rails--in length enough to make twelve steel girdles for the earth's circumference. This enormous length of rail is wonderful--we do not really grasp its significance. But the rail itself, the little section of steel, is an engineering feat. The change of its form from the curious and clumsy iron pear-head of thirty years ago to the present refined section of steel is a scientific development. It is now a beam whose every dimension and curve and angle are exactly suited to the tremendous work it has to do. The loads it carries are enormous, the blows it receives are heavy and constant, but it carries the loads and bears the blows and does its duty. The locomotive and the modern passenger and freight cars are great achievements; and so is the little rail which carries them all.