532 



BRIDGE. 



^Practice, largest arches constructed with cut stone, under the 

 y - direction of experienced persons, the sinking has 

 been scarcely discernible. In Dunkeld bridge of 

 90, Tongueland of 1 18, and Aberdeen of 130 feet 

 span, the sinking has never exceeded three inches, 

 consequently no derangement of general form or 

 of joints took place. In rivers where there is no 

 navigation, or where there is no risk from the 

 floating down of ice or timber, the centres are fra- 

 med with horizontal tie beams, and supported in 

 sundry places by frames or piles fixed in the bed of 

 the river ; and, in old bridges, corbels of stone were 

 frequently placed in each abutment, to receive the 

 beams of the centres. But when any of the before - 

 mentioned inconveniences are to be guarded against, 

 the frames are trussed so as to leave a large propor- 

 tion of the space in the middle unobstructed. In 

 this case, more art is required in framing and placinn- 

 the centres, to as to afford effectual support, and ad- 

 mit of a perfect mode of lowering and removing 

 them. The offsets of the stone work and platforms, 

 afford the most substantial abutments for the sup- 

 ports ; bat unless the bed of the river is very soft, 

 great advantage may be derived from also placing 

 frames or piles at a small distance from the body of 

 the pier ; and as in large arches this operation can- 

 not incommode the navigation, it ought to be made 

 use of. The modes of lowering or easing the cen- 

 tres practised in Britain, appears to be much more 

 simple and safe than those described by Perronet, as 

 practised in France. In France, the bridgings across 

 the frames, upon which the archstones were set, were 

 cut away, either beginning at the springing, and pro- 

 ceeding regularly upwards to the key, or otherwise, 

 by first cutting out each other bridging, over the 

 whole centre, and next, each other that was left, 

 and so alternately, until the whole were removed. 

 In Britain, the centres are set upon beams ; between 

 the centres and the beams, wedges are introduced, 

 sometimes in separate pairs across, under each frame ; 

 sometimes the wedges are formed or fixed upon one 

 piece of wood, which reaches across the whole width 

 of the soffit, passing between all the centre frames, 

 and the supporting frames or beams ; and, lastly, the 

 wedges are formed or fixed upon a piece of wood, 

 which is placed longitudinally under the foot of each 

 centre frame, and resting also on the supporting 

 frame. When the centre is to be eased, these wedges 

 are driven along each other, or the pieces of wood 

 on which the wedges are formed or fixed, are driven 

 back so that the wedges are moved along each of their 

 inclined planes into larger spaces than they had be- 

 fore occupied. In all these cases, the whole centre 

 is made to descend very gently, and may be suffered 

 to rest at any part of the operation. The mode of 

 striking the wedges is varied according to circum- 

 stances. It is, in the smaller arches, usually per- 

 formed by men with mauls striking each pair of 

 wedges. In the larger arches, such as Westminster 

 and Blackfriars, it was performed with a beam mount- 

 ed as a battering ram. The frames are placed from 

 four, six, to seven feet apart, from middle to middle, 

 and are secured with cross ties and braces. 

 When there are three arches, two centres will be 



required ; and when there are five arches, three cen- 1'racti, 

 tres will be needed. See page 54>2. v~ 



OF THE AncHES. 



The centres being placed, and properly secured, Of the 

 the setting the archstones is proceeded with. The arc ' les ' 

 masonry of the piers and abutments, near to the 

 springing, is carefully adjusted ; and it is usual, im- 

 mediately under the commencement of the curvature, 

 to lay a capping, string, or cordon : this, by having 

 a small projection, covers any trifling inaccuracy 

 which may have taken place in setting out or carry- 

 ing up the abutments and piers. If the courses have 

 hitherto been worked with horizontal beds, the up- 

 per bed ot this capping course is sometimes made to 

 suit the radius of the arch. After the general form 

 and dimensions of the arch have been determined, the 

 form and dimensions of the archstonej is of the first 

 importance, for it is upon them that the great feature 

 of bridge building depends. The nature of the differ- 

 ent forms into which they have been, may, or oui 

 to be made, have already been fully considered when 

 treating of the principles of bridges ; we shall there- 

 fore no\v only simply state various instances where 

 different sizes have been practised in great works. 



From the foregoing examples it will be seen, that 

 the French have uniformly used very deep arch- 

 stones, for their outside or headers are many of them 

 more than those here described for the body of the 

 arch ; and this circumstance no doubt,joined to their 

 wide mortar joints, led, in some measure, to the enor- 

 mous sinking of their arches. In Westminster and 

 Blackfriars, the archstones are equally deep j but by 

 the dimensions of those in Tongueland bridge near 

 Kirkcudbright,, viz. 3 feet 6 inches, it may be obser- 

 ved that no such depth is necessary; for here in an 

 arch of 1 18 feet span, erected over a river, where, be- 

 sides about 10 feet depth at low water, the tides rose 

 above 16 feet, which of course rendered it somewhat 

 difficult to fix and preserve the centering ; yet no sink- 

 ing took place to open the joints, and the whole arch 

 has ever since remained stable and perfect. Also in 

 Dunkeld bridge, the arch of 90 feet span has arch- 

 stones 3.2 deep, which are also quite sufficient. No 

 doubt the mode of managing the spandrels in both 



