394 B R I 
tenacity of the metal, the parts are fo intimately conneft- 
ed, that the accurate calculation of the cxtrados and in- 
trados, fo neceffary in (lone arches of magnitude, is ren¬ 
dered of much lefs confequence. The connefting princi¬ 
ple of thefe iron blocks Will be better undet flood by a re¬ 
ference to the annexed Plate, where fig. i represents a 
block of cafl iron, five feet in depth from A to A, and 
four inches in thicknefs; having three arms B B B, and 
making a part of a circle or elllpfis : the middle arm is 
■two feet in length from B to C, and the other two are in 
proportion. On each fide of the arms are grooves, (three- 
quarters of an inch deep, and three incites broad,) for the 
purpofe of receiving malleable or bar iron;’ and in each 
arm are two bolt-holes. D, fig. 2, reprefents two of thefe 
Mocks united together, and the joints confined to their re¬ 
spective pofitions by the bar-iron on each fide of the arms, 
as at E EE; which, with other fimilar blocks, To united 
.and bearing upon each other, become a rib. F F, fig. 2, 
are hollow tubes, fix feet long, and four inches in diame¬ 
ter, having fhoulders at each end, with holes anfwering 
to thole of the blocks. G is a block of another rib, con¬ 
nected with the former by the tubes F F, placed horizon¬ 
tally. Through the holes in tlie fhoulders and arms of 
the block and bar-iron are bolts (fattened with cotterels 
or fore-locks), as at H H HH. The blocks being united 
with each other in ribs, and the ribs connected, and Sup¬ 
ported laterally, by the tubes, as above defcribed, the 
whole becomes onemats, having the property of key-ftones 
cramped together. The blocks and tubes above Ipecified • 
are thofe uled in the conftruftion of the arch of the bridge, 
erected acrofs the river Wear, at Wearmouth, near Sun¬ 
derland, in the county of Durham, reprefented in the 
Plate at fig..3. 1 'he lizes of the blocks, tubes, and other 
parts, with the number of ribs and arms in the blocks, 
mult be fuited to the dimenlions, form, and life, of the 
arch.” 
We fhall now proceed to tranfcribe Mr. Burdon’s ac¬ 
count of the bridge at Sunderland, being the fir ft wherein 
this principle lias been adopted :—“ From the increafing 
population and trade pf Sunderland and the two Wear- 
mouths, the ancient ferry, which was alrnoft in the centre 
of the harbour, was become* very infufticient, and unfafe; 
infomuch that frequent inftances occurred of the lofs of 
lives, independent of the conftant delay and difappoint- 
ment occafioned to all defcriptions of people. Under thefe 
circumftances, Mr. Burdon, who had previoufly procured 
a turnpike-road from Stockton to Sunderland, was early 
in exprefting his willies for the accommodation of a bridge 
acrofs the Wear, as neat Sunderland as poftible. Being 
returned to parliament by the County of Durham, in the 
year 1790, he began to move in the bufinefs during the en- 
fuingyear, and an aft of parliament for a bridge was ob¬ 
tained in 1792. At firft a ftone bridge w r as propofed, of 
200 feet fpan, and eighty feet to the crown of the arch ; 
but, the plan, with the eftimate, being referred to perfons 
of lkill, the extent of expence appeared beyond all rea- 
fonable bounds ; and, upon fearching for foundations, none 
were to be found within the limits of the fpace covered 
by the tide, which flowed between rocky ffiOres, diftant 
from each other in the narrowed part about 240 feet. An¬ 
other difficulty alfo arole from the fituation being fo near 
the mouth of the river, and perpetually occupied by the 
Craft of the coal, lime, and other, trades, which could noj 
admit even a momentary interruption. From the fuccefs 
in conftrufting bridges of iron by the Colebrook-dale Com¬ 
pany, Mr. Burdon, though he difapproved of their prin¬ 
ciple, conceived the idea of making ufe of that metal ; 
adhering however to the ancient conftruftion of bridges, 
by the iubdivifion. of the parts of the arch, in the manner 
of kcy-ftones ; and, taking advantage of the duftility and 
tenacity of iron, to produce an arch of that metal; at 
leaft fifteen times lighter than a correfponding arch of 
ftone, and capable of being put together upon an ordinary 
fcaffold, inltea.l of an accurate centre, in an infinitely Ids 
fpace of time. 
D G E. 
After having caufed an experimental rib to be caft, and 
fet up, Mr. Burdon brought forward^a propofal to the 
town of Sunderland and the county, of conftrudfting a 
bridge, on his principles, over the Wear, immediately ad-i 
joining to Sunderland and its harbour. His propolition 
tvas adopted ; arid the foundation-ftone was laid on the 
north fide, on September 24, 1793. To the fuperintend- 
ance of the execution of the work, Mr. Thomas Wilfoa 
of Sunderland was appointed, through whofe indefatiga¬ 
ble perfevefance the bridge was rendered paftable, and 
opened for the accommodation of the public, on Auguft 9, 
1796. The arch is a fegment of a circle ; its fpan is 236 
feet ; the height from low water to the fpring of the arch 
is about fixty feet; and the verfed fine thirty-four; pro¬ 
ducing fo flat an arch, that fltips of 2.00 oy 300-torts, may 
pafs under it, with equal - facility, within fifty feet on ei¬ 
ther fide of its centre ; having ninety-four feet_ clear at 
i’ow water, and abundant depth in the mid-ftrearn. The 
bridge confifts of fix ribs, at five feet .diftar.ee from each 
other: the fpandrils are compofed of caft-iron circles. 
The fix ribs were put together, over the river, in the fhort 
fpace of ten days. The fuperftrnfture is of timber, plank¬ 
ed over to fupport the carriage-road, which is compofed 
of marie, limeftone, and gravel, with a cement of tar and 
chalk immediately upon the planks, to preferve then). 
The w hole width of the bridge, is thirty-two feet; and, 
on each fide, is a paved footway, having an iron palifade, 
with lamp-pofts at intervals. The weight of-the arch is 
calculated to exceed 900 tons, of which 260 tons are iron. 
Of twenty-eight parts of the iron, twenty-three are caft and 
five are wrought iron. The expence of conftrufting th.s. 
bridge amounted to upw ards of 26,000k of which 4000k 
was fubferibed by different gentlemen, and the remainder 
by Mr. Burdon. The tolls, which are the fame as thole 
of the Ancient ferry, are fubjefted by the act to pay 5 per¬ 
cent, on the capital; and all accumulations beyond thart 
are to go to difeharge the capital. The fuccefs of this 
undertaking will doubtlefs induce others to conftruft pub¬ 
lic bridges on fimilar principles', and with fimilar materials. 
Another method of conftrufting iron bridges has been 
fuggefted by Mr. John Nafli, architeft, of Dover-ftreet, 
St. James’s, London. This invention confifts of hoilmv 
frames or fquares, of wrought or caft iron, which unite 
with great exaftnefs in forming piers, arches, buttreffes, 
&c. and, when filled up with earth, fand, or gravel, be¬ 
come a folid tnafs, of great ftrength and durability. This 
invention is fecured by letters patent, bearing date Fe¬ 
bruary 7, 1797, wlterein the conftruftion is thus defcrib¬ 
ed : “ I, John Nafh, do hereby declare, that the nature of 
my invention of a new and peculiar art or method of con¬ 
ftrufting bridges of plate-iron, either wrought, caft, fram¬ 
ed, or put together,To as to form hollow bodies, maffes, or 
cubes, capable of being filled up with earth, fand, ftone, 
gravel, or other materials, to make the fame folid bodies, 
maffes, or cubes ; or, not being filled, have the femblance 
of folid bodies, maffes, or cubes ; and the manner in which 
the fame is to be performed, is particularly defcribed and 
afeertained in manner following ; that is to fay, tfie arch 
of the bridge is formed by hollow frames or boxes, each 
box conliftirg of four fides and a bottom; the fides form 
the arch-joints of the bridge, and are diminiIhed fo as to 
tend to the centre of the circle which ftrikes the arch, in 
fuch a manner that, when thefe boxes or frames are put 
together fide by fide, they form the arch of the bridge, 
the joints of which have a folid bearing throughout, like 
thofe of ftone bridges. Thefe boxes are afterwards filled 
with earth, clay, fand, or gravel, or gravel mixed with 
lime, or fand mixed with lime, or rough ftone, or rough 
ftone-mafonry, or bricks, or tree-ftone, or any other fub- 
ftance; fo that, when filled, the arch is one folid body 
caled with iron. Thefe boxes may be of caft-iron, or 
wrought-iron ; may be caft, rolled, or hammered in flat 
plates and frames, and put together. 1 hey may be caft 
without bottoms, and the bottoms put in ; or they may 
be calf with bottoms; or they may be ufed without bot¬ 
toms, 
