Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/385

EXAMPLES.] BRIDGES 337 themselves. Now, tlie result of direct experiments made with round, oval, and rectangular tubes there being pre cisely the same section and weight in all three, and, con sequently, different depths was, that the circular tube was the weakest, and the oval tube the strongest, the rectangular form being intermediate. The oval tube was, indeed, first studied with a view to its use. Its form, however, was not favourable, neither for its actual construction, nor for its connection with the suspension chains which were FIG. 126. Newark Dyke Bridge. originally intended to be used in the erection ; and practical considerations, in this case, also compelled the use of the Fio. 127. Section of Newark Dyke Bridge. rectangular tube. It must also be remarked, that the result of experiments made on round, oval, and -rectangular wrought iron tubes, when reduced to the same depth and compared, was in favour of the rectangular form, although within ordinary limits the form was not proved to be a matter of very great importance. It may be added, that this bridge has now been in use six years, that the deflection has been carefully tested, from time to time, with the utmost precision, and that not the slightest perceptible increase has taken place during that period. The care with which the painting has been attended, and the protection afforded by the roof, have also entirely preserved it from the slightest damage by oxidation ; and it is difficult to conceive that even the lapse of centuries can in any way affect such a structure, or to doubt that it will remain one of the most durable, as it certainly is one of the most remark able monuments of the enterprise of the present century." 79. Newcastle High-Level Bridge. Neivark Dyke Bridge. Newcastle Crumliii Viaduct. The High-Level Bridge at Newcastle High-Level (figs. 124 and 125, also Plate XIX. fig. 3) is a fine example Bridge of the true bowstring arch, in which there is no cross bracing. This bridge is also described at great length in the- 8th edition ; but the type cannot be recommended for imitation, being essentially more expensive and heavier than a true girder. The bridge was opened by the Queen in 1849. The design was therefore made almost at the same time as that for the Britannia Bridge, and is chiefly inter esting as showing a transitional form intermediate between the arch and beam. The bridge has six spans, each of 125 feet, and the superstructure is supported on stone piers and abutments, the height to the soffit above high water being 83 feet. The arched ribs are cast-iron, and the ties wrought iron. 4728 tons of cast-iron and 321 tons of wrought iron were employed in the superstructure. There are two roadways, the carriage roadway passing under the railway. The bridge cost 243.000. The solid or continuous plate girder soon led to the in troduction of open frames, designed on similar principles. Newark Dyke Bridge (the earliest example of a Warren Newark girder bridge) carries the Great Northern line over a D -Y ke branch of the Trent near Newark. It was erected (1851- 53) under the direction of Mr Joseph Cubitt from the designs of Mr Charles Wild. This bridge (figs. 126 and 127) consists of four inde pendent girders, viz., two for each line of railway. The roadway is beneath the girder. The top flange of each girder consists of a series of cast-iron pipes butting end to end ; the lower flange consists of wrought iron links, and the flanges are connected by diagonals forming a series of equilateral triangles, and these diagonals are alter nately struts and ties. The ties are formed of wrought iron ; and the struts of cast- iron : the length of each side of these triangles is 18 feet G inches. The length between the supports is 259 feet, and the depth from centre to centre of the joint pins is 16 feet. The clear span between the abutments is 240 feet 6 inches. The weight of iron is 244 tons 10 cwt., of which 106 tons 5 cwt. is wrought iron, and 138 tons 5 cwt. cast- IV. - 43 Fio. 128. Part of Crumlin

Viaduct.