BRIDGE. 



531 



tactice. t embrace the whole foundation* the saving of ma- 

 v sonry, when compared with the whole mass, is very 

 trifling ; and, with due deference to that able en- 

 gineer, we cannot help observing, that, in point of 

 taste, a work which is to convey an idea of durabi- 

 lity, should rather have the appearance of stability 

 towards its base, and diminish as it ascends. 



Each course of stone around the outside should 

 be laid header and stretcher alternately ; the stretchers 

 should be from 18 inches to two feet in breadth; and 

 the headers, which should be about one third of the 

 whole face, should each be from three to four feet 

 in length : their upright, or end joints, shouM be 

 correctly squared, at least one foot in from the face, 

 and in no part be more than one inch in width. The 

 interior, or filling in stones, should be of equal height 

 to the outside stones, and have their upright joints 

 not more than one inch in width : they should break 

 joint at least one foot. The first, and all the suc- 

 ceeding courses, should be laid flushed, both their 

 bed and upright joints, in proper mortar. The French 

 engineers allow from two to four and six lines for the 

 thickness of the outside mortar joints : in England 

 about an eighth of an inch, when compressed, is 

 usual. All the joints should be run full of grout, 

 where there is any vacancy after the first operation. 

 The French cramp all the outside stones with iron 

 cramps, from 15 to 18 inches in length, run and co- 

 vered with lead ; but if the masonry be composed 

 of large stones, well worked, and laid to break 

 joints properly, iron cramps appear an unnecessary 

 expence. Great care should be taken to select the 

 hardest and most perfect stones for the projecting 

 points of the piers, especially those on the upper 

 side of the bridge. The points should be carried 

 up at least to above high water mark, and at that 

 height they are usually finished by sloping them 

 back to the face of the spandrels. In some cases, a 

 projection of a circular or polygonal form is car- 

 ried up to the level of the roadway. The courses of 

 stone may Tary in thickness, 18 inches being a good 

 average. 



uhnentt. The abutments are managed in the same manner 

 as the piers, only their backing is in general made of 

 good rubble stone, laid in lime mortar. This rubble 

 work must be levelled and grouted at the height of 

 each course of square masonry ; great care being ta- 

 ken to have the whole properly bonded and connect- 

 ed together. If the bridge is wide, a buttress, or 

 counterfort, should be placed behind the middle part 

 of the abutment. This should be made of rubble work, 

 well bonded into the body of the abutment ; and 

 having, besides, thin hoop-iron, laths, or half-inch 

 boards, laid in as they are carried up. This is a ne- 

 cessary precaution in all large buttresses constructed 

 with rubble stone. 



After all the precautions which can be taken to 

 secure the foundations of piers, accidents sometimes 

 happen to the best constructed works. In the bridge 

 ef -Orleans, though conducted by the best engineers 

 in France, one of the piers sunk 18 inches, French 

 measure, although the foundation had shewn no 

 sympt-oms of being worse than the others. The 

 points did not sink with the body of the pier, but 

 both the masonry and platform broke off at these 



2 



points. The pier was loaded with 1 ,200,000lb. for Practice, 

 five months. The points were then taken down un- ' *,"*-) 

 der low water and rebuilt. This was in 1 ""!), we 

 have not heard of its sinking more. But, in 1701, 

 the water having swept away about two feet from 

 the piers, induced them to drive two rows of piles 

 quite across the river, six feet distant from the lower 

 pointa of the piers, and fill in rubble stone. 



In 1747, one of the piers of Westminster bridge 

 sunk 18 inches at one end, which caused the taking 

 down of two arches. The pier was loaded with 700 

 tons, or l,568,0001bs. It was cased round the foun- 

 dation with strong piles, to prevent any more gravel 

 running out. The pier was taken down for some 

 distance under low water, and rebuilt level ; the two 

 arches were also rebuilt j but, to lighten the pier, 

 arches were constructed in the spandrels, and the 

 same was afterwards done at Orleans. 



At Orleans, the cause of the sinking of the pier 

 was not discovered. At Westminster, it took place 

 from there being no piles under the foundation, 

 and from the ballast men lifting gravel for covering the 

 bridge too near to the foundation of the pier. This 

 circumstance ought to prove a caution to engineers, 

 never to leave unprotected, a foundation composed of 

 gravel, sand, or mud. 



OF CENTRES. 



Having carried the piers and abutments up to Of Centre;, 

 the height at which the arches are to spring, the 

 next object is to set up timber frames, usually known 

 by the name of centres. To construct and erect these 

 in a judicious manner, is one of the most masterly 

 operations in bridge building ; but as this forms also 

 a principal feature in CARPENTRY, we must refer the 

 reader to that head, under which he shall find the subject 

 fully discussed j and at present content ourselves with 

 giving drawings, and very general descriptions of 

 centres, which have been used in constructing some of 

 the principal bridges in Britain and France. See VIATIS 

 Plates XC IX. and C. XCIX. and 



In a centre, the principal objects to be kept in view, 

 are to construct and fix such a frame as shall support 

 the weight of the arch-stones, through all the pro- 

 gress of the work, from the springing of the arch, 

 to the fixing of the key- stone, without changing its 

 shape, and to admit of its being removed with safety 

 and ease. From inspecting the Plates, it will be 

 seen that the French engineers brought the timbers 

 of each frame very near together, and kept them 

 very narrow at the footing upon which they rested ; 

 whereas, in England, it is the practice to place the 

 timbers more apart, and extend the footing. The 

 comparative merits of these principles will be dis- 

 cussed when treating of CARPENTRY; but we are war- 

 ranted in observing, that the English centres have 

 succeeded best in practice. At Neuifly, previous to 

 the centre being eased, Perronet states, that the 

 arch (of 120 French feet) had sunk 13 inches, and 

 it afterwards sunk 10^ inches more, making in all 

 234 inches. At Mantz, (also 120 French feet span,) 

 before the centre was eased, the sinking was lz 

 inches, and it afterwards increased to 20. In Britain, 

 no uch imperfectious have ever been known. In (hr 



