BRIDGE. 



cases should also be taken into account, but we are 

 : led, that archstoned art, in general, made much 

 r than necessary ; and on account of the mine- 

 v weight and expense incurred by this practice, 

 we consider it our duty to draw the particular atten- 

 tion of the young engineer to this point. lie may, 

 in the instance of Llunrwst bridge, observe, that our 

 British Palladio (Inigo Jnoci,) has long ago shewn 

 us a bold example, which has stood the test of 174> 

 years. In Llanrwst bridge, the middle arch of 58 

 feet span, 17 feet rise, and 1 ! feel in width across 

 the soffit, the archstones arc only 18 inches deep. 

 We have carefully examined this arch, and can assure 

 the reader, that it is not from the peculiar excellence 

 oi the workmanship it derives any advantage. The 

 archstones vary in thicknets from 5 to 16 inches ; 

 many of them are 8 and 9 inches. There are, in some 

 instances, two headers to answer one course of com- 

 mon archstones, and in others two courses of arch- 

 stones to answer one header. The masonry of the 

 spandrels is very irregularly laid ; and as a complete 

 proof of the stability of the middle arch, from a de- 

 fect in the foundation of the west abutment, one of the 

 side arches fell, and was rebuilt in 1703, but the others 

 remained uninjured. The piers are JO feet in thick- 

 ness, and the arch springs about 3 feet above the bed 

 of the river. The road over the bridge is certainly 

 too steep, and thereby no doubt lessens the weight 

 upon the side arches ; but we are convinced, that the 

 road might be made with a declivity of 1 in 24, and 

 the same degree of lightness preserved, by construct- 

 ing the spandrels hollow, in the way which will here- 

 after be described, instead of filling them up solid 

 as they are at present. We are more particular in 

 describing this bridge, because of the authority it de- 

 rives from the celebrity of its designer, and the length 

 of time it has endured. 



From 2 feet 6 inches to 4> feet, is a good length 

 for the face or soffit of the archstones. When they 

 are longer, as the beds can scarcely ever be worked 

 and set exactly true, they are apt to break when the 

 weight comes upon them ; and when shorter, there is 

 not sufficient space to overlap or break the joints 

 properly. Each course should be of equal thickness 

 quite through between the headers. The thickness 

 of each course should be from one- third to one-half 

 their depth, and they should be chamiered or rusti- 

 cated along the bed joints, and also those of the out- 

 side heads. The beds should be worked as true as 

 possible for the whole breadth of each stone, the ne- 

 glect of which destroys every other precaution. Each 

 stone should be laid so as just to swim in the mortar, 

 and be struck with a maul two or three good blows. 

 The joints of the headers should be of equal thick- 

 ness with those of the other stones in the same course. 

 Inexperienced masons, by laying the headers with 

 thinner joints for show of fine work, frequrrtly cre- 

 ate an unequal pressure, which bursts or splinters the 

 headers, before the interior archstones come to an 

 equal bearing. 



The French cramp the headers with iron to the 

 interior archstones; but it the masonry i,s good, this 

 seems totally unnecessary, nor is it practised in Bri- 

 tain, 



In setting the archatones, care should be taken to 

 keep each course pointing in the direction of the ra- 

 diu-, ; and in order to enable the workmen to do thii 

 correctly, the tlm-krrbs of each course should be 

 marked upon the om nd it* line of direction 



upon the lower part of the beams of the same nbs. 

 Attention must also be had to carry on the courses 

 equally on each side of the centre, and also to carry 

 up masonry over the solid part of each pier in tile 

 spandrels. If this is not attended to, too much weight 

 on one side will derange the form of the centre ; and 

 there is sometimes a necessity to lay some temporary 

 weight upon the crown of the centre, until the load 

 approaches the middle. In case of more arches than 

 one, and the arches flat, care must be had not to ex- 

 pose one tide of the pier until it has a sufficient weight 

 upon it, or is guarded by resistance on the c j 

 aide. At the bridge of Mantz, by a neglect of this 

 sort, oneof the piers w;u pushed 1 ' inches out of tile- 

 upright. It was afterwards by loading the opposite 

 side, made to return '2^ inches. 



The keystones should be driven to fill their places 

 firmly, but not so as to require much force, other- 

 wise they will derange the rest of the joints, and alter 

 the shape of the arch. As soon as the keys are driven, 

 all the back and end joints of the whole arch should 

 be carefully examined ; and if from the drying of the 

 mortar, or defect of any of the atones, some vacancies 

 appear, they should be run full of mortar, and lirmly 

 wedged with slates ; the whole should then be left 

 for some time to dry and indurate. The length of 

 this time must depend upon the state of the weather, 

 the qualities of the stone, and also the mortar. While 

 this operation is going on, the masonry should be 

 brought up in the spandrels to the level of about ^ 

 of the rise of the arch. This may be of rubble work, 

 but it should be laid closely, in regular cour 

 good lime mortar, and abutting firmly against t he arch- 

 stones. The outside stones should, in the part over 

 the pier, be carried up to the same height; but im- 

 mediately adjacent to the archstones, they should be 

 stepped or racked back, and left so until the centra- 

 is removed, because.it finished close up to the b^ck 

 of the archstones, the least sinking of the arch would 

 cause a fissure. The spandrels having been carried 

 .up to the proper height, and the mortar having ac- 

 quired a proper consistence, the centre may be eased 

 and removed in the way formerly described. 



The centres, having been removed, the soffit of the 

 arch should be carefully examined, and the joints if 

 necessary, pared, cleaned out, and pointed with mor- 

 tar. The advantage arising from champhered or rustic 

 joints will now appear : they prevent the edges from 

 chipping, and cover any trifling inequality, so that it is 

 the cross joints only which require paring : the cham- 

 phered joints also give an appearance of lightness to 

 the archstones. It is sometimes the practice to lay 

 around the arch a thin course of stone, plain or mould- 

 ed on the edge, which projects a few inches below 

 the face of the archstones. This becomes nco 

 if the headers have any twist, because it admits of the 

 spandrels being set correctly straight, without shew- 

 ing the twist or other defect 



