182 



RENNIE'S BRIDGES. 



PART VII. 



Mr. Rennie also introduced a practice of some novelty 

 and importance in the centering upon which the arches 

 of the bridge were built. He adopted the braced prin- 



CENTERING OF AKCH, WATERLOO BRIDGE. [After E. Blcre.] 



ciple. The centres spanning the whole width of the 

 arch were composed of eight ribs each, formed in one 

 piece, resting upon the same number of solid wedges, 



a solid vat or tub impermeable to 

 water ; and within these, when 

 pumped clear of water, the excava- 

 tion was made to the proper depth, 

 and in the space so dug out the 

 building operations proceeded. The 

 coffer-dams for the piers were formed 

 in a similar manner, with modifica- 

 tions according to circumstances. By 

 this means the bed of the river, where 

 the piers were to be erected, was ex- 

 posed and dug out to the proper 

 depth, and the foundations were com- 

 menced from a level nine feet at 



least below low water mark. The 

 foundations there rested upon timber 

 piles from 20 to 22 feet long, driven 

 into the solid bed of the river. Upon 

 the heads of these piles half-timber 

 planking was spiked, and on this the 

 solid masonry was built every stone 

 being fitted, mortared, and laid with 

 studious accuracy and precision. The 

 whole work was done with such soli- 

 dity that, after the lapse of fifty years, 

 the foundations have not yielded by a 

 straw's breadth at any i oiiit. 



