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sider the letter published by you last Friday, in defence of Mr M--'s design for a new bridge.
Mr M---- proposes elliptical arches. It has been objected that elliptical arches are weak, and there. fore improper for a bridge of commerce, in a country where greater weights are ordinarily carried by land than perhaps in any other part of the world. That there is an elliptical bridge at Florence is allowed, but the objectors maintain, that its stability is so much doubted, that carts are not permitted to pass over it.
To this no answer is made, but that it was built for coaches; and if it had been built for carts, it would have been made stronger: thus all the controvertists a rree, that the bridge is too weak for carts; and it is of little importance, whether carts are prohibited because the bridge is weak, or whether the architect, knowing that carts were prohibited, voluntarily constructed a weak bridge. The instability of the elliptical arch has been sufficiently proved by argument, and Ammanuti’s attempt has proved it by example.
The iron rail, whether gilt or varnished, appears to me unworthy of debate. I suppose every judi. cious eye will discern it to be minute and trifling, equally unfit to make a part of a great desigo,what. ever be its colour. I shall only observe how little the writer understands his own positions, when he recommends it to be cast in whole pieces from pier to pier. That iron forged is stronger than iron cast, every smith can inform him; and if it be cast in large pieces, the fracture of a single bar must be repaired by a new piece.
The abrupt rise, which is feared from firm circular arches, may be easily prevented, by a little
extension of the abutment at each end, which will
The whole of the argument in favour of Mr.
The architrave of Perault, which has been pom-
That Mr M obtained the prize of the archi-
Dec. 15. 1759. IT is the common fate of erroneous positions, that they are betrayed by defence, and obscured by explanation; that their authors deviate from the main question into incidental disquisitions, and raise a mist where they should let in light.
Of all these concomitants of errors, the Letter of Dec. 10, in favour of elliptical arches, has af. forded examples. A great part of it is spent upon digressions. "The writer allows, that the first excellence of a bridge is undoubtedly strength ; but this concession affords him anopportunity of telling us, that strength, or provision against decay, has its limits; and of mentioning the monument and cupola, without any advance towards evidence or argument.
The first excellence of a bridge is now allowed to be strength; and it has been asserted, that a semiellipsis has less strength than a semicircle. To this he first answers, that granting this position for a moment, the semi-ellipsis may yet have strength sufficient for the purposes of commerce.
This grant, which was made but for a moment, needed not to have been made at all; for before he concludes his Letter, he undertakes to prove, that the elliptical arch must in all respects be superior in strength to the semicircle. For this daring assertion he made way by the intermediate paragraphs; in which he observes, that the convexity of a semiellipsis may be increased at will to any degree that strength may require; which is, that an ellipti.
cal arch may be made less elliptical, to be made less weak; or that an arch, which by its elliptical form is superior in strength to the semicircle, may become almost as strong as a semicircle, by being made almost semicircular.
That the longer diameter of an ellipsis may be shortened, till it shall differ little from a circle, is indisputably true; but why should the writer fora get the semicircle differs as little from such an ellipsis? It seems that the difference, whether small or great, is to the advantage of the semicircle; for he does not promise that the elliptical arch, with all the convexity that his imagination can confer, will stand without cramps of iron, and melted lead, and large stones, and a very thick arch; assistances which the semicircle does not require, and which can be yet less required by a semi-eilipsis, which is in all respects superior in strength.
Of a man who loves opposition so well, as to be thus at variance with himself, little doubt can be made of his contrariety to others; nor do I think myself entitled to complain of disregard from one, with whom the performances of antiquity have so little weight: yet in defiance of all this contemptuous superiority, I must again venture to declare, that a strait line will bear no weighi; being convinced, that not even the science of Vasari can make that form strong which the laws of nature have condemned to weakness. By the position, that a struit line will bear nothing, is meant, that it receives no strength from straitness; for that many bodies, laid in strait lines, will support weight by the cohesion of their parts, every one has found, who has seen dishes on a shelf or a thief upon the gallows. It
is not denied, that stones may be so crushed together by enormous pressure on each side, that a heavy mass may safely be laid upon them; but the strength must be derived merely from the lateral resistance; and the line so loaded will be itself part of the load.
The semi-elliptical arch has one recommendation yet unexamined; we are told that it is difficult of execution. Why difficulty should be chosen for its own sake, I am not able to discover; but it must not be forgotten, that as the convexity is i screased, the difficulty is lessened; and I know nct well whether this writer, who appears equally ambitious of difficulty and studious of strength, will wish to increase the convexity for the pain of strength, or to lessen it for the love of difficulty.
T'he friend of Mr M---, however he may be mistaken in some of his opinions, does not want the appearance
when he prefers facts to theories; and that I may not dismiss the question without some appeal to facts, I will borrow an example, suggested by a great artist, and recommended to those who may still doubt which of the two arches is the stronger, to press an egg first on the ends, and then
I am, Sir,