B III D G E. 



ing, centres, domes, &c. and we mu.it therefore refer 



the reader to that article for further 



this useful and generally interesting subject. 



InoN BRIDC; 



In the practice of bridge building, there yet re- 

 mains to be described a mode not only unknown to 

 the ancients, but unquestionably a late invention of 

 British artists. We are not perfectly certain with 

 \rhomthc proposal first originated, whether it was sole- 

 ly with the Coalbrookdale Company, or whether the 

 late Mr John Wilkinson had some share of the merit ; 



lain it is, he was very active in promoting the firat 

 iron bru! 



A thorough discussion of this subject would in- 

 volve an investigation of the principles of working 

 with iron as a material; but for this we must refer to 

 the article IKON ; yet as our readers would naturally 

 bedisappointed.ir, under I he head of Bridge Building, 

 no notice was taken of the modes hitherto practised 

 in this important change of bridge operations ; we 



::1 therefore, notwithstanding the length to which 

 we have unavoidably been led to extend the present 

 article, give a short account of the different modes 

 practised in the principal iron bridges which have 

 hitherto been constructed. 



The first, as has already been observed, was that 

 okdalo erected upon the Severn, a little below Coalbrook- 

 dale, where that river is narrow and rapid. See 

 Plate XCI. The abutments are of stone ; they are 

 brought up to about 10 feet above the surface of 

 common low water ; here they have each a platform 

 of squared freestone for ten feet breadth, which serves 

 for a hauling way, and a base for the arch to spring 

 from. Upon this platform, cast iron plates, four 

 inches in thickness, are laid, and formed with sockets 

 to receive the ribs. These plates, in order to save 

 metal, have considerable openings in them. The 

 principal, or inner ribs, which are five in number, and 

 which form the arch, are 9 inches by 6. The 

 2d row behind them, and which are cut off at the 

 top by the horizontal bearing pieces, are fr by 6 

 inches ; the 3d row are 6 by 6 inches ; the upright 

 standards behind the ribs are 15 inches by 6^ inches, 

 but they have an open space in the breadth of .5^ ; 

 the back standards are 9 inches by 64> with projec- 

 tions for the braces ; the diagonals, and horizontal 

 ties, are G inches by 4- inches, and the cast iron tie 

 bolts are iij diameter. The covering plates, which 

 are 26 feet in length, reaching quite across the bridge, 

 arc one inch in thickness. The great ribs are each 

 cast in two piecea, meeting at the keys, which, as the 

 arch is circular, 100 feet C> inches span, and 45 feet 

 rise, are about 70 feet in length. There are circular 

 rings of cast iron introduced into the spandrels, and 

 there is a cast iron railing along each side of the road- 

 way of the bridge: the weight of the whole of the 

 iron work is i37>V tons. Behind the iron work, at 

 each extremity of the arch, the abutments are carried 

 up perpendicularly of rubble masonry, faced with 

 squared stone, and the wing walls are also of the 

 same materials. 



The iron work was cast and put together in a very 



masterly manner, under the dire 

 by,of Coalbrookdale; and th<:\vln.!r watcomp 

 the year 1777- The design wa .id, 



and was, as far as the iron work goes, well ex-- i.t--.! ; 

 but being a first attempt, and placed in a situation 

 when? more skill than that of tl;< . master 



: .'quired, several radical defects are now apparent. 

 The banks of the Severn ar ;gh 



and steep, and consist of coal measures, i 

 points of which vast masses of alluvial earth li 

 down, being impelled by springs in the upper 

 the banks, and by the rapid stream of tb 

 dissolves and washes away the skirts below: 'I 1 

 masonry of the abutments and wing walls not being 

 constructed to withstand this operation, has been torn 

 asunder, and forced out of the perpendicular, more 

 particularly on the western side, where the abutment 

 has been forced forward about 3 or four inches, and 

 by contracting the span, has of course heaved up the 

 iron work of the arch. This has been remedied un- 

 der the direction of that able mason Mr John Simpson, 

 of Shrewsbury, as far as the nature of the case will 

 admit of, by removing the ground, and placing piers 

 and counter arches upon the natural ground behind 

 it. Had the abutments been at first sunk down into 

 the natural undisturbed measures, and constructed of 

 dimensions and form capable of resisting the ground 

 behind, and had the iron work, instead of being for- 

 med in ribs nearly semicircular, been made flat seg- 

 ments, pressing against the upper parts of the abut- 

 ments, the whole edifice would have been much more 

 perfect, and a great proportion of the weight of metal 

 saved. We have already stated, that one row of the 

 principal ribs formed the arch ; the two rows behind 

 are carried concentric with the inner row, until inter- 

 sected by the roadway, which passes immediately at 

 the level of the top of the inner ribs. This has a mu- 

 tilated appearance ; the circular rings of the spandrels 

 are less perfect than if the pressure had been upon 

 straight lines ; for a circle is not well calculated for 

 resistance, unless equally pressed all round. 



We consider it our duty to introduce these obser- 

 vations, in order to shew the necessity for great pre- 

 caution in similar works, and how liable first attempts 

 are to be defective ; but they derogate nothing from 

 the merit of projecting a great arch of cast iron, in- 

 troducing a material almost incompressible, which is 

 readily moulded into any shape, and which is peculiar- 

 ly applicable in theBritish isles, where the mines of iron 

 are inexhaustible and the means of manufacturing cast 

 iron unrivalled. 



The second iron bridge was built upon the same At Build' 

 river, about three miles above the former one, at a was. 

 place called Buildwas. An old stone bridge was 

 carried away by a very high flood early in 175)5, and PLATE 

 the county of Salop was obliged to restore the com- XCI I. 

 munication. Mr Telford, who was then, and is now, 

 surveyor for the public works of that county, per- 

 ceiving, that although, in a former repair, the middle 

 pier ot the four arches had been taken away, and 

 that space, as well as the two adjacent arches, con- 

 verted into one arch, yet that the water way had still 

 been too much confined ; and being aware that a few 

 years previous to thattime.theextcnsive low lands in to 

 Montgomeryshire, which formerly acted as a reservoir, 



