CHAP. \i. WATERLOO BRIDGE. 183 



supported by inclined tressels placed upon longitudinal 

 bearers, firmly fixed to the offsets of the piers and abut- 

 ments. At the intersecting point of the bearers or 

 braces in each rib, there was a cast iron box with two 

 holes or openings in it, so that the butt-ends rested firmly 

 against the metal ; and to prevent them from acting like 

 so many wedges to tear the rib to pieces when the ver- 

 tical weight of the arch began to act upon them, pieces 

 of hard wood were driven firmly into the holes above 

 described, to check the effect of the bearers or strutts of 

 the ribs ; and this arrangement proved completely suc- 

 cessful. The eight ribs were firmly connected together by 

 braces and ties, so as to form one compact frame, and the 

 curve or form of the arch was accurately adjusted by 

 means of transverse timbers, 1 2 inches wide and 6 inches 

 thick, laid across the whole of the ribs, set out to the 

 exact form of the curve by ordinates from the main or 

 longitudinal axis of the ellipsis ; and in proportion as 

 the voussoirs or arch-stones were carried up from the 

 adjoining piers, the weight which had been laid upon 

 the top of the centre to keep it in equilibrium according 

 to the form of the arch during construction, was gra- 

 dually removed as it advanced towards completion. 

 AY hen the arch was about two-thirds completed, a small 

 portion of it was closed with the centre, and the remain- 

 ing part of each side was brought forward regularly 

 by offsets to the crown until the whole was finished. 

 Each key-stone was accurately fitted to its respective 

 place, and the last portion of each, for the space of about 

 eighteen inches, was driven home by a heavy wooden 

 ram or pile-engine, so as almost to raise the crown of 

 the arch from the centre. 



About ten days after the main arches had been 

 completed, and the inverts and spandrel walls be- 

 tween them carried up to the proper height, the arches 

 were gently slackened, to the extent of about two 

 inches, so as to bring each to its bearing to a cer- 



