BRIDGE. 



48'J 



-ATE 



B, 

 SunJcr- 



LATtl 

 Cl. 



cm. 



. The span is 130 fret, tli ' v -i tin- 



ribs wliic-li bear the c -, the 



i across the soffit i.s 1 k 



ordinary low water to the soflit la .'it- te-.i. The 

 Coalbrook Dale Company performed both th 



ncl iron work by contract, and it was finished in 



tern lunli of the river is eom;> 

 matter similar to that which injured the abutnii 

 the former bridge ; but here, the foundations being 

 placed upon rock, and the masonry made of a wedge 

 form behind, the v.-hol lias continued perfect during 

 15 year*. See Plate XCI. 



The third iron bridge, in regard to time an ' 

 gresaive increase of magnitude! is that erected upon 

 the river Wear, at Sumlerh;i 1. in the county of Dur- 

 ham. It is likewise the segment of a circle, the chord 

 line being I'M} feet, and the versed sine "-t feet : the 

 height from the surface of ordinary low water to the 

 soffit is 100 feet. The merit of having this bridge, 

 instead of the ferry over the river, is chiefly due to 

 Rowland Burdon, Esq. The iron work was cast at 

 the founderies of Messrs Walkers, at Rothcrham, in 

 Yorkshire, and was erected under the direction of 

 Mr Thomas Wilson. It was opened for general use 

 in August 1TSG, and forms a magnificent feature and 

 a vry convenient passage in that part of the country. 

 See Plates XCI, XCI1I. 



The progress of this new species of bridge build- 

 ing was bold and rapid, setting out from a 'point, 



.irches constructed with atone seldom reae'u-u, I- 



and from feet. Former ex perioncc was 



left far In-hind, .mil a principle introduced, to the prac- _ 



of which we can ;>.< ' no p,,, c j over 



limit; sill'- .till bolder than any of the forego- the I'humet 



when a proposition was made by 

 , tor rebuild- 

 ing London bridge. Mr Ti-lford presented a plan of 

 one arch of cast iron, the chord of which was 600 

 fei-t, and the versed lin U This plan met 



with the approbation of the committee-, and was by 

 them submitted to the investigation of tu -us, 



most eminent in 13nta;.. ;ilic knowledge or 



practical skill. Their reports justi. ; adopt- 



ed ; but a rapid succession of political events, have 

 hitherto prevented it from being earned into effect. 



Two very neat cast iron bridges, each consisting Iron 

 of one arch of 100 feet, have been built over the river bridge* 

 Avon, at Bristol, under the direction of Mr Jessop, dtBllst "l- 

 the engineer ; and one, 150 feet span, has been de- 

 signed by, and is now building under the direction of, 

 Mr Telford, over an arm of the sea, upon one of the p LATK 

 Highland roads, in the county of Sutherland. See XCIV. 

 Plate XCIV. 



An iron bridge 85 feet span, has been built over At Boston, 

 the river Witham, at Boston, in Lincolnshire, d^ign- PLATE 

 ed by Mr Rennie. See Plate XCIV. (y) 



PART I. THEORY OF BRIDGES. 



heory of A HE construction of a magn'iicent stone bridge is 

 iridges. justly looked upon as one of the greatest performan- 

 Y""" ' ces of the masonic art : for if we compare the enor- 

 mous weight of a great arch, with the strength 

 which the cohesion of the firmest cement can give, 

 we readily admit, that it is only by the nicest adjust- 

 ment and balancing of its parts, that they are hindered 

 from instantly falling to pieces. 



Though there can belittle doubt that the Ro- 

 mans and latter Greeks had paid some attention to 

 this subject, from the beautiful specimens of their 

 architecture, which exist even in our times; yet in 

 none of their authors, either practical or scientific, is 

 the smallest light afforded us respecting the principles 

 upon which their practice was regulated. 



The architects of the middle ages, who constructed 

 those great cathedrals that are still the ornament of 

 the chief cities in Europe, and the delight of the 

 architectural antiquary, seem to have fondly indul- 

 ged in the balancing of arches. They were withoi t 

 doubt directed by maxims, which had been elicited 

 from a varied and extensive practice ; but, whatever 

 these were, they are to us unknown. None of these 

 architects, though many of them were men of learn- 

 ing, seem ever to have committed to writing, either 

 the history of any such erection, or the principles by 

 which Us construction was regulated. Nay, this 

 knowledge seems rather to have been carefully kept 

 secret, and regarded as a sort of mystery ; a cr:ft, 

 which was only to be communicated to the brethren, 



VOL. IV. t'AKT II. 



whose experience and skill had already qualified 

 them to be initiated into the mysteries cf the sub 

 lime degree. 



It does not appear, that a knowledge of this sub- 

 ject could be acquired otherwise than by experience. 

 The mathematical sciences were then little known ; 

 and we may see from the construction of the bridges 

 of that age, that the priests, who were the only ar- 

 chitects, have had in their eye rather the successive 

 vaulting of a Gothic cathedral, than to have origi- 

 nally considered of the best way of forming a perma- 

 nent and convenient road. It was only about a cen- 

 tury ago, when Newton had opened the pa.th of true 

 mechanical science, that the construction of arches 

 attracted the attention of mathematicians. S:. 

 that time, volumes have been written respecting 

 the equilibrium of arches. It has been found one 

 of the most delicate, as it is one of the most im- 

 portant applications of mathematical science. Yet, 

 with all due deference to the eminent men, who 

 have prosecuted this subject, we are much incli- 

 ned to doubt whether the greater part of their spe- 

 culations have been of any value to the practical 

 bridge builder. He is still left to be guided by a 

 set of maxims derived from long experience, and as 

 yet little improved by theory. In truth, his works 

 seldom fail even where they differ farthest from the 

 deductions of the theorist; and at all events, he finds 

 that a much greater latitude is allowable than theory- 

 seems to warrant. He is therefore surely excusable 



