INDEX AND GLOSSARY INDEX AND GLOSSARY ABBAS II, Shah of Persia, 1641-66; the Pul-i-Khaju at Isfahan dates from his time, 215. ABBOT’S BRIDGE, Bury St. Edmunds, its ecclesiastical workmanship and its double ring of voussoirs, 305 _footnote_. ABERYSTWYTH, South Wales, the Devil’s Bridge at, over the Afon Mynach, its old legend, 66, 67, 68. ABINGDON BRIDGE, Ballad of, by Richard Fannande Iremonger, dated 1458, its value to pontists, 208, 251-2. ABUTMENT PIERS, these are so strong that they act as abutments, and hence the loss of one arch does not overthrow another by withdrawing a counterbalancing thrust from one side of a pier. Perronet says: “The piers of bridges ought to be considered either as performing the duty of abutments, or as relieved of this duty by the counteraction of the collateral arches, through which the thrust is carried from abutment to abutment of the bridge. In the first case, piers should resist lateral pressure as capably as the abutments themselves, that they may withstand the side thrust of the arch-stones which tends to overturn them, and which increases by so much the more as the arches are flatter and the piers loftier. In the second case, the piers must have substance enough to carry the weight of the two half arches raised upon the two sides of each pier respectively,” together with those parts of the upper works that lie over each pier. Roman piers are abutments also, as a rule, their thickness ranging from a half to a third of the spaces between them; the effect of this great bulk both on the current of rivers and on Roman bridge-building is described on page 284. A great many bridges of the Middle Ages had abutment piers, but in many cases they were dams rather than bridges; the piers occupied far too much space in the waterways and caused terrible floods like those that happened at Lyons in the winter of 1839-40. Old London Bridge was a perforated dam (p. 220); and after her removal in 1831-2, an improvement was noted in the drainage, and consequently in the healthiness, of all the lower parts of London above bridge. So abutment piers, when they are either too thick or too numerous, are social evils. This fact was recognised by bridgemen at the beginning of the sixteenth century, when some diminution took place in the relative proportion of the piers of bridges to the spans of arches; and little by little a new routine came into vogue and displaced the abutment pier from all service. Here was another social evil, for long arched bridges with no abutment piers were unmilitary, and therefore at odds with the strategy of national defence. Not an arch could be cut without endangering its neighbouring arches. Gabriel and