quite open, or more correctly speaking, the bridge has no floor. In the case of bridges having the rails at, or above, the level of the top chords, known in the States as 'deck bridges,' parapets of any kind are seldom employed. The trusses are often placed at so short a distance apart, in the case of deck bridges, that the carriages actually overhang the sides, and a passenger on looking out of a window, is unable to discover any support beneath the train. In no bridges of two or more spans erected in the States are the trusses made continuous over a pier, each span being always treated as a bridge by itself.

One of the earliest iron trusses adopted in the States was a trellis known as Rider's bridge. Cast iron T or angle irons were employed in compression, and wrought iron bars in tension. These bridges were so slightly proportioned, that they occasionally broke down, and the Author is not aware that the plan is now adopted in new structures.

The Whipple Bridge was introduced by Mr. Syrene Whipple, C.E., in 1848, and has since been improved in detail by Mr. J. W. Murphy, C.E. Drawings are exhibited of one of these bridges of 125 feet span, with the railway supported on the lower chords (Plate 20, Fig. 3), and of another of 86 feet span, with the railway on the top. (Plate 20, Fig. 4.) These spans are measured from centre to centre of the bearings on the abutments, the clear spans between the masonry being 120 feet and 80 feet respectively. Referring to the 125 feet span, for a double line, there are three parallel trusses, the outer trusses being each 14 feet from the middle truss, measuring from centre to centre. Although the middle truss may possibly have to bear an occasional load equal to that on both the other trusses, its strength is only about one-half greater than that of either of the outer trusses. The depth of the trusses, from the centre of the top to the centre of the bottom chords, is 23 feet, or 0.184 of the span. The top chord is formed of cylindrical cast iron pipes, all of which are 8 inches in external diameter. At the centre of the middle truss these are 14 inch thick, giving 262 square inches of section; at the centre of the outer trusses they are 1 inch thick, giving 22 square inches of section; and at the ends of both trusses, they are 3 inch thick, giving 17 square inches of section. The leaning end columns are 8 inches in external diameter at the ends, 9 inches at the centre, and inch thick, and are stiffened to some extent by mouldings. The bottom chord of each truss is formed by a chain of square bars, 10 feet 5 inches long between the centres of the eyes. At the centre of the middle truss there are four of these bars, 2 inches square, giving 33 square inches, and at the ends two bars 2 inches square, giving 9 square inches. The outer trusses have four bars, 24 inches square at the centre of each bottom

chord, equal to 25 square inches, and two bars 17 inch square at the ends, equal to 7 square inches. The upright cast iron posts are all 6 inches in external diameter, and inch thick, each post being made in two lengths, faced at the ends, and abutting on each other, with a truss of four round rods 14 inch in diameter, to prevent lateral failure. The diagonals are in pairs of square rods, those at the ends of the middle truss being 2 inches square, and at the centre 1 inch square. In each outer truss there are two diagonal tension bars at the ends, each 17 inch square, the two at the centre being 18 inch square. The diagonals are formed also as eye-bars, and grasp pins 2 inches diameter in the top chord, and others 3 inches diameter where the links, or bars of the bottom chord are connected. The upright posts bear also upon these latter pins; and in the top chord the pins, passing transversely through the joint where the cylindrical pipes abut upon each other, also serve the purpose of preventing lateral failure, no other means being employed to keep the separate pipes of the top chord in line. The diagonal tension bars only cross each other in two panels on each side of the centre of the truss. The bars which are carried past the centre are 'counter rods;' they are of light section, and have slots and keys at their lower ends, for adjusting their length to any required camber of the truss. They assist also, to a certain extent, in the transmission of the strains due to unequally distributed loads. The top and bottom chords are respectively braced horizontally with transverse and diagonal bars. The railway bars are supported upon longitudinal timbers, which rest at every 10 feet 5 inches, upon a pair of transverse wrought iron rolled beams, each 9 inches deep, and weighing 90 lbs. per yard. The ends of these beams rest upon cast iron blocks forming the bases of the upright posts. The height or headway, from the top of the rails to the under side of the top bracing, is 19 feet 6 inches. The ends of the truss are not supported on rollers, the bottoms of the leaning end columns resting directly on the masonry. The weight of the three trusses, with the horizontal top and bottom bracing, but exclusive of the floor beams and longitudinal timbers for supporting the rails, is 73 tons, or nearly 6 cwt. per foot of single line. Of this weight, 43 tons are cast, and 30 tons wrought iron. Of the whole weight of the three trusses, about 32 tons, or 43 per cent., are in the upright posts and their truss rods, and in the leaning end columns and the vertical and diagonal tension rods. The rolled floor beams for the double line weigh 9 tons, the longitudinal timbers about 14 tons, and the rails and fastenings 6 tons, making a total weight for the double line of 1023 tons, equal to a little more than 8 cwt. per foot of single line. With an additional distributed load of 3,000 lbs. per running foot on each line, the tensile strain at the middle tension rods of the

middle truss would be 4.27 tons per square inch, and in the outer trusses 3.12 tons per square inch. The compressive strains in the top chords would be respectively 5-32 tons per square inch in the middle truss, and 3.55 tons in the outer trusses. These strengths, calculated for the maximum load on both lines, are not carried out in every part of the bridge; but with a train on a single line, no part is strained in tension beyond 8,000 lbs. per square inch, nor in compression beyond 10,000 lbs., and the ordinary working strain does not exceed 2 tons per square inch in tension, nor 3 tons per square inch in compression.

In the construction of the bridges just described, the tension rods are rolled in entire lengths, without weld, and from iron of the kind employed for railway carriage axles of good quality. The bars are rolled square, because that form comes out of the rolls best, when bars of unusual length are required. The eyes of all the tension rods are formed by bending the full size of the bar around a pin, the end overlapping the bar for 12 inches, and being closed by a long scarf weld. At the eyes, therefore, there is twice the section of iron that is allowed throughout the rest of the bar. The eyes are truly bored out, the pins are turned, and all the flat bearing surfaces are faced in a lathe. Bridges of this construction, up to spans of 165 feet, possess as much stability as timber trussed bridges; and the deflection of one of these bridges, of 165 feet span, on the Lehigh Valley Railroad, under a goods train of full weight was only three-quarters of an inch.

The cost of American bridges, of all kinds, is always estimated at so much per lineal foot. For a single line railway bridge, of 165 feet span, of the construction already described, the cast iron would weigh about 6 cwt. per lineal foot, and the wrought iron, including floor beams, as much more, making 12 cwt. per foot. The cost of pig iron is usually £4 108. per ton, and castings in considerable quantities £10 per ton. Wrought iron of the extra quality employed costs £23 per ton in the bar, being usually quoted at 24d. per pound At these high prices of materials, bridges containing equal weights of cast and wrought iron are sold at about £25 108. per ton in the builder's yard, although this sometimes covers the whole cost of the work erected in place. These prices, it is to be understood, are those whicn prevailed before the suspension of specie payments.

In 1861, an iron bridge was erected on the line of the Pennsylvania Central Railroad, across the Schuylkill river at Philadelphia. It had two clear spans of 192 feet each, and one pivot span, or turning bridge, 192 feet long. The construction was much like that already described as Whipple's. The truss, however, was only 19 feet deep, giving a proportion of depth to span of rather less than one-tenth. The upright posts, or struts, were

formed of wrought iron, so rolled that two bars, when put together, formed an octagonal tube. The top and bottom chords of the turning bridge were formed of wrought iron rolled beams, so that either might resist extension or compression, according as to whether the span was turned on, or off, the line of the bridge. The three spans for a single line contained an average of 5 cwt. of wrought iron, and 7 cwt. of cast iron per lineal foot. The bridge was constructed and erected by the Pennsylvania Railroad Company, and its nett cost, exclusive of masonry, was as follows:

The average cost was thus £22 138. 6d. per ton, and about £14 48. per lineal foot. The price of the cast iron is the mean of the prices of all the cast iron purchased, whether in the pig, or in castings from the founder's.

While referring to the wrought iron rolled beams employed for the top and bottom chords of the pivot span of the Pennsylvania

Fig. 1.

Fig. 2.

ΣΙ

Beam Pile.

Finished Beam.

Railroad bridge, it should be mentioned that they are rolled in a peculiar manner, and that it has been proposed to employ beams of the same kind, but of greater depth, for bridging considerable spans. Figs. 1 and 2 show sections of the beam pile and of the finished beam. The parts of the beam pile intended to be rolled out into the top and bottom flanges are first rolled into slabs by themselves, with a deep groove on one side. That portion of the pile intended to form the web is then fitted between these slabs, and the pile, thus approximating to the form of the finished beam. is then easily rolled out between rolls of moderate diameter, with comparatively little power, and without crushing, or other injury to the edges of the flanges.

The pivot, upon which the turning span of the bridge just described is made to revolve, is of a kind now extensively employed in the States for railway turntables, Fig. 3. It consists of a fixed

and of a movable cast iron disc, both being circularly grooved out to receive a number of steel rollers, each turned to the frustra of a