192 RENNIE'S BRIDGES. PART VII. 



at both ends of the arch had been put into their places, 

 they were carefully driven home to the bottom of the 

 grooves at the same time by heavy wooden rams, by 

 which means the ribs of the arches were relieved from 

 the centres and took their own bearing. In other words, 

 the arches were keyed from the abutments only, instead 

 of from the centre, as is usual in bridges of stone. This 

 was an extremely delicate and nice process, as it required 

 that the variations of the thermometer should be carefully 

 observed, in order that each operation should be carried on 

 at as nearly as possible the same degree of temperature, 

 otherwise the form of the arch would have been distorted, 

 the vertical and lateral pressure of the different parts 

 would have been affected, and an undue strain thrown 

 upon the abutments as well as the different parts of the 

 arch. But so nicely was the whole operation arranged 

 and adjusted, that nothing of the kind occurred : the 

 parts remained in perfect equilibrium ; not a bolt was 

 broken, and not the smallest derangement was found in 

 the structure after the process had been completed. 



The spandrel pieces attached to the top of the main 

 ribs were cast in the form of open diamonds or lozenges, 

 connected together in the transverse direction by two 

 tiers of solid crosses laid nearly horizontally all closely 

 wedged and firmly bolted together. In addition to the 

 transverse connecting plates cast in open squares, there 

 were also diagonal braces of cast iron, commencing at 

 the extremity of the outer rib of each arch and inter- 

 secting each other so as to form a diamond-shaped space 

 in the centre. These were also secured at their ends by 

 wedges and bolts, like the main rib pieces. After the 

 main ribs of the arches were relieved from the centres, 

 and had taken their bearing, before the centres were 

 removed from beneath them, experiments were made 

 how far they might be affected by expansion and con- 

 traction, in proportion to the different degrees of tempera- 

 ture to which the bridge might be exposed ; and for this 



