"As Mr. Cartmell told you a few moments ago, the early bridges were made of wood, and frequently required one and a half million feet of timber to make one of them. This is true of the great wooden structure over Mountain Creek on the Canadian Pacific Railroad.

"The rapid building of railroads all over this country, as we learned a few days ago, has made it necessary to build many large railroad bridges across wide rivers and deep ravines.

"One of the first of these great and costly bridges was the Niagara Falls Railway Suspension Bridge, erected in 1855. The span of this bridge is 821 feet, and the track is nearly two hundred and fifty feet above the raging green water below.

"This bridge is supported by four cables. The cables sustain two floors by wire ropes. The upper floor carries the railway track, and the lower floor forms the foot and carriage way. This bridge was carefully repaired in 1885, and seems to be in good condition for many years to

come.

"A few years after this light, airy structure was finished, George Stephenson built the great Victoria bridge at Montreal over the St. Lawrence. It is a tubular bridge built of plates of iron bolted together like a boiler. Great strength is thus secured, but the cost is enormous, and such a bridge has thirty-two acres of iron to be painted every year to prevent rusting. When Roebling finished the Niagara bridge, Stephenson said: 'If your bridge succeeds mine is a failure.' The suspension bridge has been a success for years, and it is in use to-day, for we crossed it in safety when we came to Chicago."

"What other kinds of bridges are there, Miss Gray?"

"The suspension bridge has a modern rival in the cantilever bridge. The principle of this bridge is the balancing of a part of a structure on one side of a support by a part of the structure on the opposite side of the support. I will now make a drawing to help you better to understand it.

This shows the bridge partly built. By and by the two opposite parts come together in the middle of the span."

"What prevents the bridge falling in when a train of cars passes over it ?" asked Fred.

"That part of the bridge-lever, on the land side," said Mr. Cartmell," is so much heavier than the other side that a train of cars weighing two hundred tons does not tip it at all, any more than a child would tip a 'teter' if a man were sitting on the other side. Go on, Miss Gray."

"One of the first illustrations of the cantilever bridge was the one made of steel and iron several years ago just below the suspension bridge at Niagara Falls. It is nine hundred and ten feet long, and has a clear span over the river of four hundred and seventy feet. It was built in eight months; the part over the river was built out from each side by men working on platforms suspended over

the rushing river without apparent support. People came for miles to see the bridge when it was being built. Other celebrated bridges of this kind are the Poughkeepsie over the Hudson and the Forth near Glasgow."

"I suppose," said Mr. Cartmell, "after all Miss Gray has said about suspension and cantilever bridges, it is a fact that truss bridges are cheaper and more stable for railroad traffic, when good and frequent supports can be built. In many cases trusses are made of wrought iron, but lately steel has taken the place of iron. Such bridges

are very common in the West. The Missouri and the Mississippi are frequently crossed by such structures. I presume we shall to-night cross these rivers over plain steel-truss bridges. A common style of truss bridge is

shown in the way the railroad is carried over the Snake. River in Idaho."

"What kind of bridge is the one we saw in St. Louis?" Florence asked.

"I told you," said Miss Gray, "something about this bridge when we were there. It is not a truss bridge, but consists of three steel arches, each span being over five hundred feet in length. Each span has four parallel arches, and each arch is composed of two steel tubes, eighteen inches in diameter, one above the other, twelve.

feet apart. Although not a cantilever bridge, each of the middle arches was built out over the river without support, on the principle of one part balancing another."

LESSON XVI.

OUR RAILROADS.

Part II.

MR. Cartmell and the boys then went back to the rear car and stood for a while on the end platform, observing the country.

"Do most railroads run in as straight lines as this road does, Father?" Fred asked.

"In the Central States, on the boundless prairies, it is possible for the railroads to run long distances in 'bee' lines, but in all hilly and mountainous sections of our land such a course is impossible; the line must curve in order to avoid hills and to keep the grade uniform.

"One of the best illustrations of curves is shown in the Big Loop from Georgetown to Silver Plume, on a branch of the Union Pacific in Colorado. The direct distance up the valley is one and one-fourth miles, and the elevation six hundred feet, requiring a grading of four hundred and eighty feet in a mile. By curving the line around in the form of a spiral the length was increased to four miles, and the grading was reduced to one hundred and fifty feet per mile. I will show you sometime a plan and a profile of this wonderful road."