THE AKCH. 



131 



The bridge had, in fact, to trust largely to friction at the 

 haunches, where the "line of thrust" passed from ring to 

 ring and, eventually, out of the arch into the backing. 



If this arch had been built in the true form, the necessary 

 thickness (as before described) would have been throe 

 bricks, and if a fourth were added as a margin of safety, 

 the arch would have been four bricks, or three feet thick all 

 round; but, as built, the arch is 5 feet 3 inches at the 

 crown, and from that point getting gradually thicker till it 

 is 7 foot 9 inches at the two-thirds point before alluded to, 

 and it then springs from a block of brickwork 20 feet thick, 

 with an immense basis carried down to the chalk 16 feet 

 below the surface of the ground. 



Notwithstanding this strength of work, the arch followed 

 the centres the first time they were eased, and a good 

 portion had to be rebuilt, so that there was a great doubt 

 in the public mind if the bridge ivoitld stand, until the 

 question was set at rest by the centres (which had been 

 slacked for some time) being all blown into the Thames by 

 a heavy gale of wind. 



The great fault in the construction of this bridge was, with- 

 out doubt, the building of the arch in riwjs of brickwork- 

 it would otherwise have been more than amply strong. 



Alluding once more to economy and strengtli in centering, 

 this bridge forms a good example of what has been said 

 before on this subject. It would be absurd to erect entire 

 self-supporting centres for these arches. The necessary 

 opening for the passage of boats is very small, as shown 

 by the small arches in the road bridge just above ; and it 

 would make bettor, as well as cheaper, centering to have it 

 supported from several rows of piles in each case, for the 

 reasons exi)lained above. 



In conclusion, I may say that I think Brickwork has not 



