BRIDGE, 



the otliffi- two are in proportion. On each fiJe of the arms 

 arc grooves (three quarters of an hich deep, and three inches 

 broad) for the p-.irpofc of receiving malleable or bar-iron, 

 and in each arm are two bolt holes, D. iV;-. 2. reprefents 

 two of thefe blocks placed together, and the joints tonliucd 

 to thiir refp'.itivo politions by the bar-iron on each fide of 

 tlie arms as at E, E, E, which, with other iiiEibr blocks fo 

 united and bearing upon each other, become a nb. /Vf. -;. 

 and F, F, Jig. z. are hollovv tubes iix feet long, and four 

 inches in diameter, having (lionlders at each end, with lioles 

 anfvvcring to thofj of the blocks ; O is a block of another 

 rib coTineded wiih the former by the tubes F, F, placed ho- 

 rizontally. Through the holes in the fhoulders and arms of 

 the block and bar-iron are bolts, fallened with cotterels or 

 forelocJvS, as at H, H, H, H. The blocks being united 

 with each other in ribs, and the ribs connefled and fiipportcd 

 laterally by the tubes as above defcribed, the whole btcom.es 

 <jne mafs, havilig the property of key Hones crampe<i toge- 

 ther." This extrail ferves to explain the more minute parts 

 of the conllruiilion : a few words more will defcribc the 

 ibmclure itfelf. 



The bridge confifls of a fi.igle arch, whofe fpan is 23C) 

 ■feet ; and as the fpringing Hones at each fide projeft two feet, 

 the whole opening is 2.\.o feet. The arch is a fegment ot a 

 circle of about 444 feet diameter, its verfed fine is 34 feet, 

 and the whole height from low water about ico feet, ad- 

 mitting vcfTels of from two to three hundred tons burthen 

 to pafs under, without firiking their mafts. A fcries of one 

 hundred and five blocks form a rib, and fix of thtfe ribs 

 compofe the breadth of the bridge. The fpandrels, or the 

 fpaces between the arch and the roadway, are filled up by 

 call-iron circles, which touch the outer circumference of the 

 arch, and at the fame time fupport the roadway, thus gra- 

 dually diminifhing from the abutments towards the centre of 

 the bridge. There are alfo diagonal iron bars, which arc 

 laid on the tops of the ribs, and extended to the abutments 

 to keep the ribs from twilling. The fnperllruiSure is a 

 flrong frame of timber planked over to fupport the carriage- 

 road, which is compofed of mail, lime-ilone, and gravel, 

 with a cement of tar and chalk immediately upon the 

 planks, to preferve them. The whole width of the bridge 

 is 32 feet. The abutments are maffes of almofl folid ma- 

 lonry, twenty-four feet in thick nefs, forty-two in breadth at 

 bottom, and thirty-feven at top. The fouth pier is founded 

 on the foliJ rock, and rifesfrom about twtnty-two feet above 

 the bed of the river. On the north fide the ground was not 

 fo favourable, fo that it was neceflary to carry the founda- 

 tion ten feet below the bed. The w'eight of the iron in this 

 extraordinary fabric amounts to ■160 tons ; 46 of theie are 

 Bialleable, and 214 call. The entire expence was 2;,oocl. 



From this account of the bridge aerofs the Wear, the 

 attentive reader will fee much to admire in its conftruclion : 

 it is not, however, totally free from defcdts. We conceive 

 that the fpandrels are very improperly filled up. It is true, 

 that it is done in fuch a manner as is exceedingly light and 

 pleafing to the eye ; but the iron hoops may, we think, be 

 eafily comprtifed at the points of contaft, and changing 

 their (liape will oppofe very little rcfiilance. As the arch 

 forms fo fmall a portion of a circle (about 64! degrees), the 

 weight at the fpring of the arch need not, according to the 

 theory of equilibration, be double to that at the crown, 

 to lupport, without danger of riling, any preffure arifing 

 from the mafs of the ilructure itfelf: but in fo flat and 

 light an arch, an overload on any pait mull have a great 

 tendency to bend it, and conlequently tend confiderably 

 to break it, at a dillant part, with all the energy of a 

 long lever.: we think, therefore^ that a better form might 

 . Voi. V. ■ 



liavc been adopted tlian wIiM'haj biscn put in praftice »t 

 WcHrinonth bridge. 



The third iron bridge is that over the Severn atBuildwa«, 

 about two miles above Coalbrook Dale. An old llouc 

 bridge of three narrow arches having been carried away by a 

 high flood in 17';5. the prcfent iron bridge was planned ar.d 

 built by the Coalbrook U:ile Company, under the fupcrin- 

 tend:mce of Mr. Thomas 'I'elford, the county furvcyor, ni 

 1796. It coiifills of a fingle arch, 1 ]o feet in fpan, the 

 rife from the fpringing to the fofiite being 27 feet; and a* 

 it was thought neceflary to keep tlie roadway as low as pof- 

 (ible, the outllde ribs are made to go up as high as the rail, 

 iiig ; they are connedtcd with the ribs that bear the co- 

 vering plates, by means of pieces of iron dovetailed in thr 

 form of king-polls. The [ilates which compofe the covering- 

 over the lower ribs, are call with deep (launches; they arc 

 laid clofe to each otlier, and form an arch of thcmfelves. 

 Theie fide nbs, or arches, would have added much more to 

 the llrength of the bridge than they now do, had the mate- 

 rials been of a fubllance that would not expand or contraft ; 

 but that not being the cafe, they, in warm weather, when 

 they expand, rather tend to derange the other parts of the 

 bridge than ftrengthen it; and the appearance of the wliole 

 is bv no means pleafing. 



About the fame time that the bridge at Buildvvas wr,» 

 erefted, an iron bridge was thrown over the river Teme in 

 Herefordfliire. Its parts were fo flender and iil-difpofed, 

 that no fooner was the wooden centre taken away than the 

 whole tumbled into the river. 



The fplendid example of the bridge at Wearmouth gave 

 an impulle to public tafte, and caufed an emulation among 

 artills, which has produced many examples and more pro- 

 jefts of iron bridges. The Coalbrook Dale Company have 

 conllrufted leveral, among which is a very neat one over the 

 river Parrot at Bridgewater. Mr. Wilfon, the engineer, 

 employed by Mr. Burdon, has alfo built feveral, and has 

 lately finillied a very elegant one over the river Thames, at 

 Staines, which is by far the moll complete in defign, as well 

 as the bell executed, of any that has hitherto been erefted. 

 This bridge confifts of a fingle arch, 181 feet in fpan, and 

 16 feet 6 inches in rife, being a fegment of a circle of 

 4S0 feet. The blocks, of which the ribs are compofed, 

 are fimilar to thofe in the Wearmouth bridge, except that 

 thefe have only two concentric arcs inllead of three, as at 

 the latter. The arcs are call hollow, and the blocks con- 

 nected by means of dowels and keys ; thus obviating the 

 great defeft obferved at Wearmouth, of having fo much 

 hammered iron expofed to the aclion of the air. Four ribs 

 form the width of the arch, which are connefted together by 

 crofs-frames. The fpandrels are filled in with circles, which 

 fupport a covering of iron plates an inch thick : on thij 

 is laid the roadway 27 feet wide. Two hundred and fc- 

 venty tons are the weight of the iron employed in the brido-e, 

 and three hundred and thirty of the roadway. 



Fer further praiR;ical details on the conftrudlion of bridges, 

 the reader is referred to the articles Foundations in IValei; 

 Caisson, CorFERD.tM, &c. 



Bridges, Theory of. In confidering tlie theory of 

 bridges, the firit objefls of enquirj' are, the nalare of an arch, 

 and the principles on which depend its liability and perma- 

 nence. It will be feen, by referring to the article Arch in 

 this di(£lionary, that we have adopted the opinion of thofc 

 mathematicians who conceive that an arch is kept in equi- 

 librium, or from falling, by the weight or vertical preflure 

 of the fnperincumbent wall or mafs. The principles on 

 whi.h they proceed, have now obtained the WMn^(A the thcoij 

 of Equiiitratian, 



Rr li 



