40 TECHNOLOGY. 
ings. When it is not possible to support the centring from below, it must be: 
made self-supporting, and can only rest upon the pillars at its ends. This 
problem is one of importance and difficulty, and in large bridges requires. 
an architect of greatexperience. Pl. 7, jig. 18, is the centring contrived by 
Perronet for the very flat arches of the Neuilly bridge. /%g.16 is the 
centring constructed by Rennie for the new Waterloo bridge in London ; the 
bearing points of the ties and struts are all in iron shoes, to enable them to 
resist the great strain put upon them. /%g. 23 represents Westminster 
bridge in process of building, together with its centrings. The latter are 
supported upon a great number of wedges, that any particular portion — 
requiring it may be tightened, and ultimately to facilitate the removing — 
of the centrings when the work is completed. 
As the wedges are very easily lost or misplaced, Elmes contrived for 
London bridge a species of screw wedge, of which a representation is given 
in jig. 15: the wedges m and m were moved by the screws J, and by this 
means the whole centring was capable of being raised, and ultimately of being 
lowered ; d are the shores or supports resting upon the heads of the piles. 
In jig. 20 are seen the derricks used in laying the stone, also the trucks which 
bring them to the work upon the temporary working frame; in jig. 20 the 
stones are seen slung in can hooks; jigs. 21 and 22 represent the ordinary 
lewis, used where the stone is hung from the centre. A hole, ¢, enlarged at 
the bottom is cut in the stone, and the wedge, a, is inserted ; the two cheek 
pieces, 6, are then put in, and as the wedge, a, cannot be withdrawn by a 
straight pull, the stone is raised by the hook, e. To withdraw the lewis it is 
requisite only to take out the pieces, 6, and the whole is loosened. 
Formerly the voussoirs of bridges were all of the same height (pl. 7, jigs. 
2,19, and 17, a side view of the Nydeck bridge in Berne), the extrados or 
outer surface of the arch being parallel with the entrados or inner surface, 
as in the Pons Senatorius at Rome. More recently the surfaces of the 
stones in the vicinity of the key-stone were made horizontal, as in the Pont 
Royal in Paris, the Neuilly bridge, and many others of recent times. At 
present the voussoirs are made to increase from the key-stone gradually to 
the springing line (jigs. 3, 16, and 18), for the purpose of enlarging the 
bearing surface of the arch upon the pier. For lightness the bridge is 
sometimes filled in with minor arches as seen in pil. 7, fig. 18. 
B. Wooden Bridges. 
Wooden bridges are characterized by the arrangement of the timbers 
which support the roadway over the openings to be bridged. 
In the simplest form of wooden bridges the roadway is supported upon 
piles driven into the bed of the river, or upon stone piers, either with or 
without trussing or framing. 
In all wooden bridges of large span the roadway is suspended from 
trussed frames or wooden arches. 
In pl. 8, figs.1-8, is represented the bridge over the Rhine at Schaffhausen ; 
this was one of the most celebrated wooden bridges ever built. It was planned 
and constructed in 1757 by a common carpenter, Ulric Grubenmann, and 
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