536 



B 11 1 D G E. 



Practice. If the carnage way is to be paved, there should be 

 ' - ii~ "' laid upon the covering of the spandrels, and over the 

 top of the arches, a bed of gravel mixed with loam, 

 about from 12 to 18 inches in thickness, worked 

 with water into the consistence of mortar. When this 

 has become moderately dry and firm, squared whin 

 paving stones, about four inches square, and six to 

 eight inches in depth, are to be set and well beat, 

 making a curve across the road of four inches in 

 24 in breadth ; and that curve shovild be terminated by 

 sinking four inches more in the distance of two feet 

 from the inclined plane, which has been formed along 

 the outer edge of the curbing stones. 



If the roadway is to be made with gravel only, it 

 is necessary to lay it.22 inches in depth in the middle, 

 and 18 inches near the sides : It should contain a small 

 mixture of loam, so that when worked with water, it 

 shall consolidate and afterwards exclude water. There 

 should still be about 18 inches in breadth, on each 

 side next the before mentioned inclined planes, paved 

 jwith small squared whinstones ; because by forming 

 the roadway a little convex across, and with a decli- 

 vity of 1 in 2i lengthwise, it is meant that the 

 greatest proportion of water which falls on the bridge 

 shall run along each side. 



In small bridges, where there are no cavities in the 

 spandrels, it is necessary to fill them up with coarse 

 gravel with a mixture of loam, working it with water 

 as put in ; and if this simple operation is carefully 

 performed over the arches, the trouble and expense 

 of coating with cement, and other substances, may 

 safely be spared. 



The water which falls upon the roadway of the 

 bridge, must be conducted beyond the extremity of 

 the wing walls, and be there introduced into covered 

 drains, or open paved sewers, and be afterwards car- 

 ried off in the most convenient direction. 



In or adjacent to towns, walls or quays are usual- 

 ly carried from the abutments along the banks of the 

 river ; and even in the country it is frequently found 

 to be advisable for protecting the abutments ; but 

 as the description of this sort of works falls more 

 properly under that of wharfs, we shall refer the read- 

 er to the articles HAKBOUR, and INLAND Naviga- 

 tion. 



We have known bridges, whose arches were of 

 considerable size, constructed with bricks. In this 

 case, it is customary either to have the whole of the 

 piers, or at least the upper points cased with stone ; 

 but if the points are circular, and bricks made of 

 a suitable shape, and outside bricks laid headers in 

 British cement, the work will be sufficiently substan- 

 tial. It is necessary that the bricks for the whole 

 bridge be made of good clay, well prepared and tem- 

 pered, be burnt hard, and laid with thin beds of mor- 

 tar, but to be properly flushed as they are laid, and 

 grouted afterwards. It is likewise necessary, after 

 the arches are closed, that the centres be suffered to 

 remain until the mortar has acquired a considerable 

 degree of consistence. It will greatly improve the 

 work, if laths or hoop iron are laid in as it advances. 



In respect to the formation of arches, although it 

 has been fully treated under the head of Theory, 

 yet we cannot help here observing, that the mode 

 adopted by Perronet at Neuilly has many advanta- 



ges ; for while the main body of the arch has a rise 

 of about one-fourth of the span, the outside headers, " 

 by being :\ flat segment, gives an appearance of light- 

 ness, and in high floods suits the contraction of the 

 vein of water entering the arch. 



We have been minute in treating the Practice of 

 bridge building, considering it of importance to the 

 young engineer, to be thoroughly informed of the 

 most improved modes adopted in different countries by 

 experienced persons. He will thus be enabled to judge 

 how far all, or anyof them, are applicable to the works 

 he is engaged in ; or he may draw from them hints, 

 which may lead him to improve upon all former prac- 

 tice ; and, at all events, this will prevent the waste of 

 time in contriving modes already acted upon. We 

 also conceive, that candid foreigners may profit, by 

 being made acquainted with the British manner of 

 conducting the various difficult operations connected 

 with bridge building; for by carefully considering the 

 accounts given by the ablest French engineers, it will 

 be evident, that perhaps one-third pan of the materials 

 and labour they have hitherto used, may be sav;d, 

 and their defects avoided by adopting mod;--, practi- 

 sed in Britain. We freely acknowledge and itimire 

 the portion of science which they have bestowed 

 upon those important works. We are greatly in- 

 debted to them for so minutely registering each ope- 

 ration, and for so candidly describing their errors and 

 defects. In Britain, we have no such correct jour- 

 nals of similar works published, and very few we sus- 

 pect preserved. The cause of this, we conceive to 

 originate in the French bridges having been entirely 

 under the direction of its government, who employed 

 men of liberal education, and from whom the officers 

 at the head of their department required regular and 

 minute details of each operation, from the commence- 

 ment to the completion of every work ; and for this 

 purpose, they were no doubt furnished with a suffi- 

 cient number of superintendants and clerks, who also 

 acted agreeably to forms previously arranged. Where- 

 as in Britain, with a very few exceptions, these useful 

 works have been constructed at theexpence, and un- 

 der the direction of particular, and frequently very 

 limited districts, communities, or individuals, whose 

 chief object has, in general, been economy. For it, 

 they have had recourse to every means that ingenuity 

 could devise, both with regard to the plans and per- 

 formance. The effect of this has been to create a 

 great competition amongst all persons who have had 

 experience, or any idea of such works. The desire 

 of the competitors to have their proposals accepted, 

 Jias led them to recur to every project which could 

 reduce the expence ; and though from the want of k no w- 

 ledge and desire for economy in those who decided 

 on the merits of the proposals, very frequently infe- 

 rior plans have been adopted, yet, from the sagacity 

 and good management of the practitioners, we have 

 Scarcely ever known, in any work of magnitude, 

 defects similar to those described in the most perfect 

 of the French bridges : indeed such defects would 

 have rendered the works so inadmissible, that the con- 

 tractors would have been obliged to take down and 

 reconstruct them. 



This mode of competition, joined to the quantity 

 of floating capital always in this country, ready to be 



