In respect to the formation of arches, although it has been fully treated under the head of Theory, yet we can not help here observing, that the mode adopted by Per ronet at Neuilly, has many advantages : for while the main body of the arch has a rise of ainut one-fourth of the span, the outside headers, by being a flat segment, gives an appearance of lightness, 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 expe rienced persons. Ile will thus be enabled to judge how far all, or any of 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 practice ; and, at all events, this will prevent the waste of time in con triving modes already acted upon. We also conceive, that candid foreigners • may profit, by being made ac quainted with the British manner of conducting the va rious difficult operations connected with bridge build ing ; for by carefully consiciering the accounts given by the ablest French engineers, it will be evident, that per haps one-third part of the materials and labour they have hitherto used, may be saved, and their defects avoided by adopting modes practised in Britain. We freely acknowledge and admire the portion of science which they have bestowed upon those important works. We are greatly indebted to them for so minutely register ing each operation, and for so candidly describing their errors and defects. In Britain, we have no such correct journals of similar works published, and very few we suspect 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 mi nute details of each operation, from the commencement to the completion of every work ; and for this purpose, they were no doubt furnished with a sufficient number of superintendants and clerks, who also acted agreeably to forms previously arranged. Whereas in Britain, with
a very few exceptions, these useful works have been constructed at the expence, and under the direction of particular, and frequently very limited districts, com munities, or individuals, whose chief object has, in gene ral, been economy. For it, they have had recourse to every means that ingenuity could devise, both with re gard to the plans and performance. The effect of this has been to create a great competition amongst all per sons who have had experience, or any idea of such works. The desire of the competitors to have their proposals accepted, has led them to recur to every pro ject which could reduce the expence ; and though from the want of knowledge and desire for economy in those who decided on the merits of the proposals, very fre quently inferior 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 bridge's : indeed, such defects would have rendered the works so inadmissible, that the contractors 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 ap plied in any way which promises profit, has, of late years, led to an extreme upon this principle, and ren dzred caution necessary in selecting, even at an appa rent increase of expence, the best qualified of the com petitors. of whom we have frequently known upwards of twenty for one project.