The new bridge at Poughkeepsie has three of these cantilevers, connected by two fixed spans, as shown in the illustration (pg. 36). The fixed spans have horizontal lower chords, and really extend beyond each pier and up the inclined portions, to where the bottom chord of the cantilever is horizontal. At these points the junctions between the spans are made, and arranged in such a way, by means of movable links, that expansion and contraction due to changes of temperature can take place. The fixed spans are 525 feet long. Their upper chord, where the tracks are placed, is 212 feet above water. These spans required stagings to build them upon. These stagings were 220 feet above water, and rested on piles, driven through 60 feet of water and 60 feet of mud, making the whole height of the temporary staging 332 feet, or within 30 feet of the height of Trinity Church steeple, in New York. The time occupied in building one of these stagings and then erecting the steel-work upon it was about four months. The cantilever spans were erected, as shown in the illustration on page 37, without any stagings at all below, and entirely from the two overhead travelling scaffolds, shown in the engraving. These scaffolds were moved out daily from the place of beginning over the piers, until they met in the centre. The workmen hoisted up the different pieces of steel from a barge in the river below and put them into place, using suspended planks to walk upon. The time saved by this method was so great that one of these spans of 548 feet long was erected in less than four weeks, or one-seventh of the time which would have been required if stagings had been used. [Illustration: Pope's Cantilever in Process of Erection. (From his "Treatise on Bridge Architecture.")] At the Forth Bridge, all the projecting cantilevers will be built from overhead scaffolds, 360 feet above the water. It contains two spans of 1,710 feet each. When spans of this length are used, the rivets become very long--seven inches--and it would be impossible to make a good job by hand riveting. Hence a power-riveter is used in riveting the work upon the staging. A steam-engine raises up a heavy mass of cast-iron, called "the accumulator;" the weight of this in descending is transmitted through tubes of water, and its power increased by contracting the area of pressure, until some twenty tons can be applied to the head of each rivet. One rivet per minute can be put in with this tool. * * * * *