44 TECHNOLOGY. 
10 show the open work of arches upon a larger scale ; jig. 11, the vertical 
bars uniting the three ribs of the arches, whilst the latter are firmly stayed 
together by the transverse rods (jig. 12). In the sunken panels of the ribs, 
a (fig. 10), over the vertical joints between the voussoirs, lie wrought-iron 
plates (fig. 13), which are screwed to the ribs and bind the voussoirs firmly 
together. 
A peculiar construction of iron bridges was introduced in 1837 by a 
French architect, Polonceau, who built upon this plan the Carrousel bridge 
in Paris. In this construction the strength of iron is united to the elasticity 
of wood. The arches of the bridge consist of hollow cast-iron tubes filled with . 
wood and imbedded in asphaltum. //%g. 20 is a view of this bridge, which 
consists of three arches, with stone abutments and piers. Upon each askew- 
back are five cylindrical boxes or springing-plates, into which the arch ribs 
are set (jig. 32). These ribs consist of elliptical tubes composed of two 
pieces, one of which is seen in jig. 26 in various views and sections. The 
two halves united are shown upon a large scale in fig. 25, in which the 
plank filling is seen. Between each layer of wood is a thick layer of 
asphaltum. Thesemi-cylinders are united together by screws (pl. 9, fig. 25) ; 
fig. 21 shows the foot of the arch upon a large scale. The large rings 
between the arch and roadway are seen in section in jig. 29 and in elevation 
in jig. 30, from which the connexion between the rings is apparent; jig. 31 
is a horizontal section of one of these rings; jig. 28 is a view from above 
of the ribs of the arch, the roadway being removed, and showing the 
diagonal braces, x, and the transverse braces, L; jig. 22 is a vertical section 
of the arch of the bridge; jg. 28 shows the connexion between the diagonal 
and transverse braces with the ribs of the arch. The stringers 6 (jig. 24) 
are borne in iron shoes (jig. 27) by the five arch ribs and the supporting- 
rings D; the stringers are made in two parts screwed together, and the two 
outside stringers are covered with iron plates, o (jig. 24). Above these 
stringers come the transverse sleepers of the roadway, a, carrying the con- 
sole plate G, the elevated foot-walk being supported by the tringle x, the 
support 7, and the bearer. The sleepers also carry the iron plates, e, which 
support the foundations, r and «eG, with the road-covering above. Jig. 22 
shows the whole roadway in section. 
For laying the arch ribs, co, a peculiar centring was constructed (jig. 33), 
resting upon two temporary piers ; the ribs were supported upon this centring, 
on blocks and wedges, where they were filled with the wooden plates, 
screwed together, and secured in place. 
Of an entirely different character from the bridges described above are 
those in which the roadway is suspended from chains or wire cables, 
stretched from the top of towers or tall piers. The suspension system, though 
new in Europe, has been long known in India and America ; to the English, 
however, is due the credit of having perfected the system, which rests upon 
the properties of the so called catenary, that is the curve which a thread 
takes when suspended at both ends and left to itself. If now in lieu of the 
thread, two chains or wire cables be imagined suspended at a distance from 
each other equal to the width of the bridge, with the roadway suspended 
624 
