OF CYLINDRICAL VAULTS.

7

so, by the Pont-y-ty-Prydd * over the Taafe in Gla­ morganshire , built by the Welch mason Edwards;this arch, from its size and elevation above theadjoining trees, is compared to a rainbow; and wemay add, in the words of the son of Sirach , “ thehands of a skilful artist have bended it.” This archapproaches very nearly in hardiness of constructionto that of Brioude .

The bridge of Santa Trinita t, over the Arno,affords perhaps the most scientific example of vault-ing in Europe , and is not less an object of beauty.Drummond states, that the curves of the arches arecycloidal, if so, they must be from an elongated cy-cloid. Ferroni shews them to be pointed at thevertex. (See Tomo xiv., della Societa Italiana dellaScienze.)

The form of the arches of the bridge of Neuilly X,built by Perronet, is a false ellipse. The upper partof the arch is formed of an arc of a circle 320 feetdiameter; the height of the key-stone is 63 inches.During the construction it sunk or flattened, so thatit became an arc of a circle, whose diameter wouldbe 520 feet; whence it is manifest that an arch mightbe built of stone from Saillancourt, a stone of ordi.nary strength, with a height at the key-stone of 63inches, 520 feet on the chord line; and the flyingbridge of China , which the Jesuits state to be built

* The chord of the arch of the Pont-y-ty-Prydd is 140 feet:the arch is an arc of a circle whose diameter would be 175 feet.

f The transverse diameter of the middle arch of the bridge ofSanta Trinita at Florence is 95 feet, the semi-conjugate 15 feet.There is an engraving to a large scale of this bridge in vol. iii. ofBouchard’s edit, of Ruggieri’s Scelta d’Arch.

X The transverse diameter of each of the five arches of thebridge of Neuilly is 128 feet.

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