ENGINEERING. 



283 



bridge by the aid of chains. "Water is supplied 

 to the central press by an accumulator, at a 

 tension of 270 atmospheres. 



The Severn bridge is expected to be open in 

 the spring of 1879. The cylinders for the last 

 pier have been put in place. The weight of 

 iron used in this bridge is about 7,000 tons. 

 The total length of the bridge is 1,387 yards, 

 including the masonry, viaduct, and swing- 

 span. The width of the river at the place 

 where it is crossed by the bridge is 1,186 

 yards. 



There have been frequent ominous predic- 

 tions of the rapid decay and impending fall of 

 the famous Britannia tubular bridge, con- 

 structed by Stephenson across Menai Straits. 

 An engineering critic has answered them by 

 an estimate that the bridge will hold, with 

 proper care, without any extensive repairs, 

 for at least 150 years, while the parts which 

 are so wasted by corrosion as to imperil the 

 structure can be successively removed and 

 new plates riveted on ; and the whole bridge 

 can be gradually renewed in this manner with- 

 out altering its form or efficiency. The iron- 

 work of the top and bottom cells, which would 

 be the most difficult to repair, is the portion 

 of the structure which is least liable to cor- 

 rosion, and a recent examination of these 

 parts shows them to be in a perfectly sound 

 and unimpaired condition. Experiments have 

 proved that iron which is subject to repeated 

 vibrations corrodes more slowly than iron 

 which is quiescent. 



A flat arch of 75 feet span and 7 feet 6 

 inches rise in the center, forming a bridge re- 

 cently built over a railroad cutting in London, 

 is entirely composed of concrete. The con- 

 crete in the arch is 3 feet 6 inches thick in the 

 center, and increases in thickness toward the 

 haunches ; these abut on skew-backs composed 

 likewise of concrete. The material is composed 

 of 6 parts of gravel and 1 of Portland cement. 

 It was carefully laid on in mass over a close 

 frame of boarding set upon the centering and 

 inclosed at the sides. 



A viaduct over the Douro River in Portugal, 

 recently constructed, is 1,129 feet in length 

 between the faces of the abutments, supported 

 by iron trestles, and over the river is sustained 

 by a central arch of 512 feet span. An arch 

 of this size was adopted on account of the 

 depth of the river and the thickness of the 

 clay beds at its bottom. On account of the 

 unusual dimensions of the arch, it was un- 

 advisable to employ rigid tympanums, which 

 greatly increased the complexity of the calcu- 

 lations. The arch was therefore given suffi- 

 cient rigidity to resist strains by making it 32 

 feet thick at the key, while the extrados and 

 intrados of the arch were made to converge at 

 the abutments in order that the ends might 

 rest on two supports. The form of the arch 

 was therefore that of a crescent, within which, 

 bracing the extrados and intrados, are vertical 

 and transverse pieces in the form of St. An- 



drew's crosses. To resist the force of the 

 wind, the base was made 48 feet broad, the 

 two sides of the arch approaching each other 

 at the summit, where their distance apart is 

 only 12-8 feet, the width of the roadway ; the 

 plane of the two arches is therefore consider- 

 ably out of the vertical. They are connected 

 transversely by horizontal traverses and frames 

 joining the crosses and posts of the arches, and 

 by timbers joining the arches themselves. The 

 roadway is supported on each side of the river 

 by a metallic pillar resting on the spandrel of 

 the arch, and by other similar pillars in the 

 valley. The roadway is attached to the arches 

 in such manner that they can move without 

 disturbing it. The pillars are entirely of 

 laminated iron, cast iron having been consid- 

 ered insecure. As it was impossible to erect 

 scaffolding in the river, the difficult work of 

 utilizing the structure on either bank to sup- 

 port the massive arches while erecting them 

 was attempted. The horizontal girders, which 

 support the roadway over its whole length, 

 were run out some distance beyond the iron 

 piers on either bank, and the portions of the 

 arch, as they were successively built out from 

 each shore, were held up by a system of wire 

 cables made fast to different parts of the frame- 

 work of the girders and iron pillars, which 

 were themselves anchored in position by iron 

 ropes which were fastened in the ledge of 

 natural rock where the bridge terminates on 

 the Oporto side, and in a mass of masonry con- 

 structed for the purpose on the other bank. 

 The different bays of the arch were thus sus- 

 tained, the wire ropes being successively at- 

 tached to the outer ones as they were com- 

 pleted. The iron parts were brought in barges 

 and hoisted into position by cranes and shear- 

 legs. The work was commenced in January, 

 1876, and was completed in 20 months. The 

 engineers were Eiffel & Co., of Paris. When 

 tested with a train of 35 tons weight, running 

 at the rate of 19 miles an hour, the deflection 

 was 0*6 inch in the center, and 0*4 inch at the 

 haunches ; a stationary load of 18 cwt. per 

 lineal foot caused a deflection of 0*394 inch, 

 the estimated deflection having been 0.355 

 inch. 



The raising of the great Egyptian obelisk, 

 called Cleopatra's Needle, and setting it upright 

 upon its pedestal, was a work requiring novel 

 engineering expedients and apparatus. The 

 obelisk was hauled on its peculiar raft along- 

 side the Adelphi Steps of the Thames Embank- 

 ment in London, and hauled upon a timber 

 cradle ; it was then lifted and at the same time 

 moved forward by hydraulic jacks, and then a 

 little to the side, until its center rested upon 

 the center of the pedestal. The iron cylinder 

 in which it had made its long sea-voyage was 

 then knocked off and replaced by an iron 

 jacket, which incased the central part of the 

 obelisk for 20 feet of its length. The jacket 

 was furnished with protruding arms resem- 

 bling the trunnions of a cannon, which rested 



