184 KENNIE'S BKiDQES. PART VII. 



tain extent. This was effected by driving back the 

 wedges upon which the ribs of the centres rested, by 

 means of heavy wooden rams attached to them, so 

 that they could swing backwards and forwards with 

 great facility when any external force was applied to 

 them ; and this was done by ropes worked by hand- 

 labour. After the first striking or slackening, the 

 arches were allowed to stand for ten days, when the 

 wedges were driven back six inches further. After ten 

 days more the wedges were driven back sufficiently to 

 render the arch altogether clear of the centering. By 

 this means the mortar was firmly imbedded into all the 

 joints, and the arch came gradually to its ultimate bear- 

 ing without any undue crushing. In order to ascertain 

 whether any change of form took place, three straight 

 lines were drawn in black chalk on the extreme face of 

 the arch previous to commencing the operation of striking 

 the centre, one horizontally in the centre of the vous- 

 soirs forming the crown, and two from the haunches of 

 the arch, each intersecting the first line at about 25 feet 

 on each side of the keystone ; so that if there had been 

 any derangement of the curve or irregular sinking, it 

 would at once have been clearly apparent. After the 

 centres had been removed, it was found that the sinking 

 of the arches varied from 2i to 3i inches, which was as 

 nearly as possible the allowance made by the engineer 

 in designing the work ; the whole plan being worked 

 out with admirable precision and accuracy. 



The method of fixing and removing the centres was 

 entirely new; being precisely the same as was after- 

 wards followed by Mr. Eobert Stephenson in fixing 

 the wrought iron ribs of the Con way and Britannia 

 bridges, that is, by constructing them complete on a 

 platform adjacent to the river, and floating them between 

 the piers on barges expressly contrived for the purpose. 

 They were then raised into their proper places by four 

 strong screws, 8 inches in diameter and 4 feet long, 



