¥796.] 
tlemen, and a fectetary, to correct and 
arrange the Journal of the fociety, and 
fuperintend the publication of thofe pro- 
ccedings which the fociety may with to | 
fubniit to the public. 
8. That the ftock, implements of huf- 
bandry, &c. fhall remain the property of 
the fubferibers.’ : 
eee 
To the Editor of the Monthly Magazine. 
PudoS Ty Giaaas 
A® your Provincial Intelligence for 
** the prefent month will undoubt- 
edly include a relation of the open- 
ing of the new Iron Bridge, at Sun- 
derland, fome account of the princi- 
ples upon which that ftupendous edi- 
fice is conftrucéted, may not be unaccept- 
able to your readers, and the rather, as 
they are totally different from thofe 
which have been ereéted of the fame 
materials, in other places. ‘The princi- 
ple of this bridge proceeds entirely 
upon the idea of rendering the arch in- 
finitely lighter than: it could have been 
made in ftone, by means of the great 
voids which caft irom will permit, and 
the fimplicity with which that metal will 
adopt any form. ‘The blocks, which are 
caft to ferve as arch-ftones, are made of 
the annexed form and dimenfions : 




= = 

Their’ thicknefs is no more than four 
inches, and their weight about 4 cwt. 
Thefe are kept in their places, and made 
to bear accurately upon eachother, inthe 
manner of keyftones, by bars of wrought 
iron, which run along grooves (marked 
by fhades) on each fide of the blocks, 
and are bolted through, at equal dif- 
tances, to braces of caftiron, patling ho- 
rizontally between the ribs ; of fix of 
which, placed at five feet from each 
other, the bridge confifts. The wrought 
iron is common Swedifh, or Ruffhan bar 
iron, which may be taken out, and re- 
placed, if neceffary; but there is every 
Sunderland Iron Bridge. 
54 
reafon to believe, that an application of 
coal, tar, and pounded charcoal, to the 
iron, in a heated ftate, will forma fpecies 
of japan, which will refift the weather 
for many years. ‘The wrought iron is 
as three to twenty-five of cait iron in the 
cae ufed.; its price as 34 to 12. 
Phe particular defeription of the car- 
caffes, &c. upon which the centre was 
laid, and the various other very ingeni-= 
ous -ontrivances, du ing the courle of 
this very fpirited undertaking, will be 
laid before the public in a work now pre- 
paring for the particular information of 
profeiional men. In the mean time, 
a beautiful perfpective view of the bridge 
has juft been publifhed at Newcaftle, by 
Mr. Robert Johnfon, a very ingenious 
young artift, and Mr. A. Hunter, en- 
graver, price 5s. which will, perhaps, 
convey a better idea of it to the public 
in general. From this view, it appears, 
that it is a fmall fegment of a very large 
circle ; for though its {pan is 236 feet, 
the verfed fine, or perpendicular to the 
chord, is only 34 fcet. The fpandrils, of 
courfe, do not require much filling up ; 
and this is done in the lighteft manner, | 
by iron circles, placed upon the ribs, and 
gradually diminifhing in fize, from the 
abutments towards the centre. ‘The 
whole is braced, arid tied together at» 
top by timbers, on which planks are laid, 
to carry a kind of lead roof, with the 
earth and materials of the road above 
it. Irs height is 100 feet above high 
water mark; fo that fhips of confiderable 
burthen are continually pafling under it ; 
of courfe the piers are 76 feet of folid 
mafonry. Stupendous limeftone rocks 
are the foundation of that on the fouth 
fide ; the rifing ground being fomewhat 
more diftant from the river on the north, 
the interval is filled up with ample ware- 
houfes, over which the road is carried. 
What may have been the expence of it, 
your correfpondent has not the means of 
being informed ; but if it be confidered, 
‘that, ina great meafure, both the in- 
vention, the expence, and the rifk, belong 
to one individual (Rowland Burdon, efy. 
one of the members for the county) who 
cannot, in the utmoft ultimate fuccefs, be 
any farther benefited than by the re- 
ceipt of that common intereft for his 
money which he might have had without 
tifkany where, it muit certainly be clafled 
among the moft public fpirited under- 
takings of ancient or modern times. 
¥. &. 
On 

