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CONSIDERATIONS

ON THE

PLANS offered for the Conftruction of BLACK-FRIARS BRIDGE.

In THREE LETTERS, to the PRINTER of the GAZETTEER.

LETTER I.

SIR,

THE

Dec. 1, 1759.

HE Plans which have been offered by dif ferent architects, of different reputation and abilities, for the Conftruction of the Bridge intended to be built at Black-Friars, are, by the rejection of the greater part, now reduced to a fmall number; in which small number three are fuppofed to be much fuperior to the reft; fo that only three architects are now properly competitors for the honour of this great employment; by two of whom are propofed femicircular and by the other elliptical arches.

The question is, therefore, whether an elliptical or femicircular arch is to be preferred ?

The first excellence of a bridge built for commerce over a large river, is ftrength; for a bridge which cannot ftand, however beautiful, will boast its beauty but a little while; the ftronger arch is therefore to be preferred, and much more to be

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preferred, if with greater ftrength it has greater beauty.

Those who are acquainted with the mathematical principles of architecture, are not many; and yet fewer are they who will, upon any fingle occafion, endure any laborious ftretch of thought, or harass their minds with unaccustomed investigations. We fhall therefore attempt to fhew the weakness of the elliptical arch, by arguments which appeal fimply to common reason, and which will yet ftand the teft of geometrical examination.

the top

All arches have a certain degree of weakness. No hollow building can be equally ftrong with a folid mass, of which every upper part preffes perpendicularly upon the lower. Any weight laid upon of an arch, has a tendency to force that top into the vacuity below; and the arch thus loaded on the top, ftands only because the ftones that form it, being wider in the upper than in the lower parts, that part that fills a wider space cannot fall through a space less wide; but the force which laid upon a flat would prefs directly downwards, is difperfed each way in a lateral direction, as the parts of a beam are pushed out to the right and left by a wedge driven between them. In proportion as the ftones are wider at the top than at the bottom, they can lefs eafily be forced downwards, and as their lateral furfaces tend more from the centre to each fide, to fo much more is the preffure directed laterally towards the piers, and fo much lefs perpendicularly towards the vacuity.

Upon this plain principle the femicircular arch may be demonftrated to excel in ftrength the ellip

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tical

tical arch, which approaching nearer to a strait line, must be conftructed with ftones whofe diminution downwards is very little, and of which the preffure is almoft perpendicular.

It has yet been fometimes afferted by hardy ignorance, that the elliptical arch is stronger than the femicircular; or in other terms, that any mafs is more strongly supported the lefs it refts upon the fupporters. If the elliptical arch be equally ftrong with the femicircular, that is, if an arch, by approaching to a strait line, lofes none of its ftability, it will follow, that all arcuation is useless, and that the bridge may at laft, without any inconvenience, confift of stone laid in ftrait lines from pillar to pillar. But if a ftrait line will bear no weight, which is evident at the first view, it is plain likewise, that an ellipfis will bear very little; and that as the arch is more curved, its ftrength is increased.

Having thus evinced the superior strength of the femicircular arch, we have fufficiently proved, that it ought to be preferred; but to leave no objection unprevented, we think it proper likewise to observe, that the elliptical arch must always appear to want elevation and dignity; and that if beauty be to be determined by fuffrages, the elliptical arch will have little to boast, fince the only bridge of that kind has now ftood two hundred years without imitation.

If in oppofition to these arguments, and in defiance at once of right reafon and general authority, the elliptical arch fhould at laft be chofen, what will the world believe, than that fome other motive than reafon influenced the determination? And fome degree of partiality cannot but be fufpected by him, who has been

been told that one of the judges appointed to decide this question, is Mr. Mr, who having by ignorance or thoughtleffness, already preferred the elliptical arch, will probably think himself obliged to maintain his own judgment, though his opinion will avail but little with the publick, when it is known that Mr. S-pf-n declares it to be falfe.

He that in the lift of the committee chofen for the fuperintendency of the bridge, reads many of the most illuftrious names of this great city, will hope that the greater number will have more reverence for the opinion of posterity, than to disgrace themselves, and the metropolis of the kingdom, in compliance with any man, who, instead of voting, afpires to dictate, perhaps without any claim to fuch fuperiority, either by greatness of birth, dignity of employment, extent of knowledge, or largenefs of fortune.

IN

SIR,

LETTER II.

Dec. 8, 1759.

N questions of general concern, there is no law of government, or rule of decency, that forbids open examination and publick difcuffion. I fhall therefore not betray, by a mean apology, that right which no man has power, and, I fuppofe, no wife man has defire to refufe me; but fhall confider the Letter published by you laft Friday, in defence of Mr. M's defign for a new bridge.

Mr. M propofes elliptical arches. It has been objected that elliptical arches are weak, and therefore improper for a bridge of commerce, in a country where greater weights are ordinarily carried

by

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 fo much doubted, that carts are not permitted to pafs 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 ftronger: thus all the controvertists agree, that the bridge is too weak for carts; and it is of little importance, whether carts are prohi bited because the bridge is weak, or whether the architect, knowing that carts were prohibited, voluntarily conftructed a weak bridge. The inftability of the elliptical arch has been fufficiently 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 fuppofe every judicious eye will discern it to be minute and trifling, equally unfit to make a part of a great defign, whatever be its colour. I fhall only obferve how little the writer understands his own pofitions, when he recommends it to be caft in whole pieces from pier to pier. That iron forged is ftronger than iron caft, every smith can inform him; and if it be caft in large picces, the fracture of a single bar must be repaired by a new piece.

The abrupt rife, which is feared from firm circular arches, may be eafily prevented, by a little extenfion of the abutment at each end, which will take away the objection, and add almoft nothing to the expence.

The whole of the argument in favour of Mr. M

is only that there is an elliptical bridge at Florence, and an iron baluftrade at Rome; the bridge is owned to be weak, and the iron balustrade we confider as mean;

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