A classic tradition says that the huge stones in the Pont du Gard were joined together by iron clamps. Is this true? Each iron clamp, if any were used by the masons, connected a voussoir to an interior archstone.[66] From time to time the Romans employed iron rods bent at the ends and fastened into stones with molten lead; such rods have been discovered among the ruins of a Roman bridge over the North Tyne, at Chollerford, near Hexham. This was a bridge with a wooden superstructure, probably, as no voussoir has been found among the litter of pier-stones. The Pont du Gard is very tall; it soars, and to a height that exceeds forty-seven metres. The first tier has six arches, the second has eleven, the third has thirty-five. In the middle tier the length is 257 m. 90 cm. Note, too, that the architectural centre of the design is determined by the rocky channel of the Gardon; we find it not in the centre of the bridge but on the north in the arch under which the river flows. It is the biggest arch of all, with a span of 25 m. 30 cm., while the neighbour on each side is narrower by nearly six metres. The other bays of the first arcade dwindle in span to 15 m. 75 cm.[67] As to the centre of the second tier, it corresponds with that of the first, for the largest vault is above the river; it carries four little arches of the third arcade, while its companions support only three. Some critics see nothing more than the unequal size of these arcades, when the real point is to find the architectural centre, whence the composition radiates, majestic and imperious. The topmost arches and their crowning dignify the whole structure with a completeness akin to that which is given to a long range of columns by a fine entablature and cornice. And we must note the symbol of prosperity--a phallus--carved twice in low-relief on the Pont du Gard. On the western side it graces a springing voussoir in the third arch of the second tier; and there is another on the keystone of the greatest arch, where the river passes. Here the emblem is a double phallus, and when it is touched by sunlight it looks as young as hope, not as uncertain as prosperity.