The use of ribbed vaulting in English churches dates from the twelfth century; it came to England from France. Yet Scotland, the historic friend of France, used it very rarely in bridges; perhaps only once, in the famous Old Bridge of Dee near Aberdeen, which dates from the beginning of the sixteenth century. Mr. G. M. Fraser, a Scotch pontist, tells me that he has looked in vain throughout Scotland for another example. Old Stirling Bridge, and the Brig o’ Doon, and the Auld Brig o’ Ayr, and Devorgilla’s Bridge at Dumfries, all finely historic and various, have plain arches. On the other hand, ribbed arches are fairly common in North English bridges. One of the best examples architecturally is the graceful single arch that Sir Walter Scott loved in Twizel Bridge, that enabled Lord Surrey to outflank the Scotch before the battle of Flodden Field.[28] Why the frugal Scotch were unattracted by a new and thrifty way of building I cannot explain, unless by supposing that they loved convention even more than a hard economy. Viollet-le-Duc estimates that in _arcs-doubleaux_, or ribbed arches, builders use a third less of tooled and clavated masonry; hence a great saving not of cost only, but of dead weight also. And there were other economies. An _arc-doubleau_ is the simplest form of ribbed vaulting: at given intervals in the building of a vault a concentric arch is supposited, or the vault itself at intervals is made much thicker than at others. In Poitou, where ribbed bridges were studied by Viollet-le-Duc, the intervals between the ribs are filled in with flagging under the roadway; and with this material--or with ashlar--the spandrils above the ribs are packed. When flagstones are used, and rain-water filters down from the roadway, no harm is done; the wet trickles away through the joints of the flagstones, without causing the haunches of an arch to throw out saltpetre: a mishap that occurs often when arches are unribbed. I am writing here with the mind of Viollet-le-Duc, who makes two other valuable statements: first, that ribbed bridges are notable in Poitou; next, that they seem to belong to the beginning of the thirteenth century, or perhaps even to the end of the twelfth. Now it was in 1214 that King John invaded Poitou without success; fifteen years later Henry III misconducted an expedition to the same province; and again in 1242 he landed in Poitou to be thrashed at Taillebourg. His aim, like that of John, was to win back the Empire of Henry II. May we then suppose that ribbed bridges came to us from Poitou? Certainly the mind of England during the first half of the thirteenth century was drawn towards the seaward provinces of France.