MEANS OF COMMUNICATION. 39° 
for the exterior only is cut. So soon as the pier has reached the level of the 
spring of the arch, the coffer-dam is destroyed, and the water allowed to 
come to the pier. 
It often happens that the stream where the bridge is to be built isso deep 
or so rapid that no coffer-dam can be built, or that the ground below is of 
such a nature that timber framing is not necessary ; in such cases another 
method of laying the foundations is adopted, the pier being built in large 
water-tight boxes, or chambers, called caissons, which are afterwards sunk 
upon the spot where the pier is to stand. 
These caissons must be sufficiently large not only to hold the pier, but 
also to accommodate all the workmen who may be employed upon it at a 
time. Pl. 7, jig. 11, is the plan of the bottom of the caisson, which shows 
the grating or framework on which the masonry is laid. Around the edge of 
this bottom double water-tight plank walls are built, between which 
tenacious clay is rammed, and so arranged, that on the completion of the 
work the sides may be separated from the bottom. /%g. 9 is a front view 
of the caisson, jig. 8 is a vertical section. In this caisson, as seen in jig. 12, 
the pier is built, the caisson sinking as the work proceeds until it reaches 
the ground below. /%g. 10 shows a caisson grounded upon the bottom, 
already prepared for it by divers; here is also seen the manner in which 
the caisson is stayed to the pier as it sinks into the water. %g. 7 shows 
the manner in which the piers of Westminster Bridge, London, were built 
in caissons. When the pier has reached the springing line of the arch, or 
at least is above the surface of the water, the sides of the caisson are loosed 
from the bottom, to be used again in the same manner. The arch of the 
bridge is now to be commenced, but previously the centring, which is to 
support the vowssovrs or separate stones of the arch, must be built. The 
centring must be sufficiently strong to sustain the weight of the whole struc- 
ture without sinking, until the key-stone is put in, which binds the whole 
together, and for large bridges must be constructed with extreme care and of 
great strength ; otherwise, as the two ends of the arch are built, the weight of 
the materials depresses the centring at these points, causing it to rise in con- 
sequence in the centre, and the arch of the bridge to become higher or more 
pointed than was intended. To prevent this, even with the best constructed 
timbering, the top of the centre should be loaded with stones as the work 
proceeds at the ends, and the tendency of this part to rise thus counteracted. 
Centrings are either supported from below, or are self-sustaining, as seen 
at jigs. 16 and 13. Piles may be driven, or small stone pillars may be 
raised for the purpose, upon which the centring is built; jig. 14 shows an 
arch of London bridge, with one of the centrings resting upon the piles, a, 
upon which are placed the tie-beams, 6, suspended by the trusses, d, ¢ ; 
the other struts of the centring, 7, also rest upon the piles, which thus 
carry the whole weight of the centring and bridge until the key-stone is in. 
Figs. 19 and 20 represent a supported centring of a bridge built in Berne in 
1842; here the support is afforded partly by stone pillars, partly by piles, 
which carry the temporary bridge for the support of the laborers and 
materials ; the construction of the centring itself is apparent from the draw- 
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