MEANS OF COMMUNICATION. 43 
spans the arch may be cast in a single piece, in larger spans it may be cast 
in many pieces and united by bolts, or voussoirs may be cast and set after 
the manner pursued with stone bridges, or the roadway may be hung upon 
chains or even wire cables. Desaguiliers and Garrin, in the commencement 
of the 18th century, proposed the building of iron bridges, but the idea was 
first carried out in England. The first iron bridge was that over the Severn 
near Colebrookdale, erected in 1779. Pl. 9, fig. 1, is a view of one half 
of the arch; jig. 2, a cross-section; jig. 3 is a plan with the roadway 
removed ; jig. 4, a plan of the springing plates on which the arch rests ; 
jig. 5 shows the fastenings of the diagonal braces, 5, of jig. 3 with the 
main arches; jig. 6, the connexion between the cross-braces and the arch 
ribs. These figures are so clear that they require no further explanation. 
The bridge is one arch of 100 ft. 6 inches span and 45 ft. high from the 
level of the springing-plates to the middle of the soffit. The height from 
ordinary low water to the springing-plates is 10 ft., making the whole 
height from low water to the soffit 55 ft. 
The bridge was designed and executed by Abraham Darby and formed 
a new era in bridge building. The form of the intrados is nearly a semi- 
circle and consists of five ribs, upon each of which rests one of the longi- 
tudinal stringers which support the roadway. Upon these stringers are 
placed iron plates 24 inches thick, which support the road-covering, consist- 
ing of clay with broken iron cinders. 
Soon after the completion of the above bridge the second iron bridge 
was built, three miles higher up the Severn. The engineer, Thomas Telford, 
a county surveyor, introduced the principle of suspending the bridge upon 
two large ribs, one on each side of the bridge. The span is 130 ft., the 
versed sine of the ribs which bear the covering plates is 17 ft. and the 
breadth across the soffit is 18 ft.; the height from ordinary low water to 
the soffit is 34 ft. %g. 14 is a side view of this bridge; jig. 15 is a cross- 
section of the same; jig. 16, a plan of the springing-plates ; jig. 17 shows 
the connexion and bracing between the main arch B and the intersecting 
arch c, by the upright braces @ and diagonal braces 0; jig. 18 shows the 
connexion of the railing with the bridge road, and jig. 19 shows the 
connexion of the two arches at the crown. It will be perceived that 
the auxiliary arch is for the purpose of supporting the main arch at its 
weakest point, the latter being suspended by iron straps to the auxiliary 
arch. The cost of the bridge, including the abutments, was £6000 
sterling. 
The third bridge in regard to time and progressive increase of magnitude 
was that over the Wear at Sunderland, in the county of Durham. The 
arch is the segment of a circle, the chord being 236 ft. and the versed sine. 
or height of the crown of the intrados above the level of the springing line 
34 ft., so that the largest ships may pass beneath it. 
It is of the boldest construction, and is put together very differently from 
those already described, the arches being composed of open-work boxes or 
gratings, which take the place of the voussoirs in a stone arch. 7g. 7 is a 
side view of the bridge, and jig. 8 a perspective view of one end; jigs. 9 and 
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