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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[March, 



struction, this difference becomes a source of considerable trouble and 

 annoyance. 



It is objectionable to have to add to the length of pieces when 

 cast, and to avoid this, the patterns are usually made full long, 

 thereby allowing for the greatest possible contraction of the metal ; 

 so that generally speaking, a certain portion of each casting has to 

 be removed by the chisel or otherwise, to get the pieces to their 

 proper length. Notwithstanding these precautions, there will be oc- 

 casional wasters, which may be properly or improperly patched up, 

 or which if thrown away, give rise to extra expense and delay. 



When the constructor has plenty of time at his disposal, he pro- 

 ceeds with still further caution, by casting the principal pieces, and 

 having them put together; after this, the pattern of the remaining 

 length is corrected, and sent to the foundry with the certitude, that 

 the casting will come in pretty well ; thus, by dint of precaution, delay, 

 and expense, the work is got through to this stage, and if the mason 

 work is prepared with the same care and precision, all will be found 

 to come in very well; but a stone pier has been known to be a little 

 out of its proper position, either in consequence of an error or the 

 settling of the work upon its foundation. When this takes place, it 

 becomes requisite to let the cast iron into the stone work on one side 

 of the pier, and to place a thickness of metal between the pier and 

 the cast iron on its other side. All these imperfections are only felt 

 during the construction, they do not at all diminish the strength or 

 interfere with the durability of the work, but generally speaking, all 

 those who have been engaged in cast iron bridge building, will have 

 bad to exercise their ingenuity, not only to correct such evils, but at 

 the same time to proceed in such a manner, that they may not leave 

 an indelible trace on the face of the work. There is, therefore, no 

 doubt but that the facility of extending, or diminishing, the chord 

 line or the versed sine of the arch, would on many occasions be of 

 considerable advantage. 



The keys placed at each end of the segments, of the tubular vous- 

 soir, remove all the above mentioned objections, for the segments 

 being kept rather short, the space will have to be divided amongst 

 some 10 or 12 joints, so that three or four inches, more or less, in the 

 total length of the arch, will only require the wrought iron keys to be 

 made a little thicker or thinner than they were originally intended to 

 be, and as they are not made until after the arc has been placed, no 

 extra expense will be incurred, or time lost. 



The division of the arc into so many pieces, offers another advan- 

 tage, as, by reducing the weight of each separate piece, the whole can 

 be moved about, and managed with great facility, and without requir- 

 ing such powerful tackle, or such strong centering, as is generally 

 employed. When the number of segments comprised in an arc are in 

 their place, a wood model is made for each key, and the keys are 

 forged and fitted in their places, without being immediately driven 

 home ; plumb bobs being suspended from the joint of the tubular rib, 

 it becomes very easy to set the whole in a perfect line, by driving the 

 keys on either side as may be required ; and by making the keys 

 sufficiently long, the height of the arc can be regulated with the ut- 

 most facility and precision. The keys being driven, and the whole of 

 the tubular arc in its proper place, the bolt holes, those of one fianch 

 having been cast in, are drilled in the opposite one, and the bolts 

 placed and tightly screwed up, attention being paid at the same time 

 to the plumb bobs, as the effect of screwing up the nuts may be to 

 cause a deviation in the line of the arc, which is again easily rectified 

 by means of the keys, and the bolts cannot otherwise affect the form 

 of the arc. 



Openings were reserved from distance to distance along the upper 

 joint of the voussoir, for the purpose of introducing melted bitumen, 

 which, when the bridge was finished, was done, in order to fill up the 

 space remaining between the lamellated wooden rib, and its cast iron 

 covering. This bitumen being intended, by setting when cool, to form 

 a compact mass of the whole arc, with a view to increase its rigidity. 

 The wooden arc, as I have already said, greatly facilitated the 

 erection of the tubular voussoir, and when standing alone, previous to 

 the application of the latter, it had a wonderfully light and elegant 



appearance ; but it becomes a question whether, in reality, it is at all 

 requisite to the solidity of this kind of bridge, its flexibility being so 

 great that it cannot in any way be expected to come to the assistance 

 of the cast iron, which, if accidentally forced out of its original 

 position, by any extraordinary lateral strain (the only one that could 

 affect it) would break long before the internal wooden rib could offer 

 any useful resistance to the strain. It also remains to be ascertained 

 whether the wood thus confined in the tubular voussoir, will not be 

 very subject to decay, notwithstanding every precaution that has been 

 taken to preserve it. 



The annular system of spandrils, adopted by M. Polonceau, forms 

 another remarkable feature in this construction ; they combine strength, 

 with lightness, and give an elegant appearance to the bridge. Their 

 circular form renders them elastic; they spring under the load, so that 

 the vibratory action, communicated to the roadway, and the upper 

 side of the annular support, is neutralized ere it can arrive at [the 

 main rib, which, therefore, as I have already observed, maintains the 

 most rigid firmness, under the heaviest load. 



At the same time that the vibratory action is destroyed, they pre- 

 sent also the advantage of distributing the load acting at a particular 

 point on the road, over a considerable length of the main rib of the 

 arch, as will be shown by the diagram. 



Let us suppose three rings a, b, c, placed between a beam d, and an 

 upper platform; and that a weight W be placed on the platform im- 

 mediately over the ring 6 ; under such circumstances the vertical di- 

 ameter of the ring 6, will be shortened, and its horizontal diameter 

 will be lengthened in the same ratio ; half the increased length of the 

 horizontal diameter of b, will be taken from each of the horizontal 

 diameters of a and e, and added to their vertical diameters, thus 

 raising the platform immediately over the centres a and c, and by 

 increasing its resistance, will remove a portion of the load W from 

 the point b, to the centres a and c, thereby distributing it along the 

 beam d. 



As a proof of the advantage of this mode of construction in point 

 of economy, it will be sufficient to mention, that although the length 

 of this bridge between the buttresses, is nearly 500 feet, it was com- 

 pleted for £40,000, including every expenditure; and that, in a 

 country where metal is very high priced. I can conscientiously 

 affirm, that every precaution was taken, and every outlay made, that 

 could be required to obtain good workmanship, and a solid construc- 

 tion. The whole of the tubular ribs were moulded in dry sand, and 

 cast in second fusion, and the wrought iron employed is of the very 

 best quality for the purpose. 



When the bridge was finished, it became necessary, according to 

 the government regulation, to test its strength by distributing a given 

 load over the entire surface of the road way, the government officers 

 in the meanwhile, minutely inspecting the work, to ascertain whether, 

 under the strain, any defect would become apparent. This, in France, 

 is an invariable rule adopted as a legislative measure of public 

 security; and, until you can present a favourable report from these 

 officers, it is impossible to obtain permission to open the bridge. We 

 commenced by making an observation in the morning early, to deter- 

 mine the positive height of each arch, previous to any load having 

 been placed on the bridge. The load (about 400 lbs. avoirdupois 

 upon every square yard of surface) was then laid on, and in the after- 

 noon we found that, under the load, each arch stood higher than it did 

 in the morning without the load. 



The morning (October) had been very cool, the sun coming out 

 later in the day heated the metal, and increased its length, so that the 



