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When builders of old began to rear masonry they repeated in stone or brick the forms they had constructed in wood. Accordingly the lintels of their doors and windows were flat. It was a remarkable step in advance when the arch was invented, probably by a bricklayer, spanning widths impossible to horizontal structures. A flat course of stone or brick presses downward only; an arch presses sidewise as well as downward. It is this sidewise thrust, calling into play a new resource, that gives the arch its structural advantage. In modern masonry the boldest arch is that of the bridge at Plauen, Germany, with its span of 295¹⁄₄ feet. Of pointed arches the chief sustain the walls of Gothic cathedrals; it was to counteract the outward thrust of these arches that external buttresses were reared, either solid, as at St. Remy in Rheims, or flying, as at Notre Dame in Paris. The Saracenic arch, offering more than half of a circle, is not so strong as the Roman arch, but it has a grace of its own, fully revealed in the Alhambra, and in the incomparable mosque at Cordova. A chain of small links, a watch-chain, for example, freely hanging between two points of support strikes out a catenary curve; this Galileo suggested as the outline for an arch in equilibrium; it is adopted for suspension bridges.

Longest stone arch in the world, Plauen, Germany.

Church of St. Remy, Rheims, France.

Section across buttressed choir.

Curve of suspended chain.

Dam across Bear Valley, San Bernardino County, California.

“The arch,” says Mr. William P. P. Longfellow in “The Column and the Arch,” “was the great constructive factor in the architecture of the Roman Empire; it added enormously to the builder’s resources in planning, and to his means of architectural effect. It gave him the means of spanning wide openings, and when expanded into the vault, of covering great spaces; it habituated him to curved lines and surfaces. Helped by it, and spurred by the new wants of the complex Roman civilization, he enlarged the scale of his buildings and greatly increased the intricacy of their plans. He used his new combinations with a boldness and fertility of invention that have been the wonder of the world from that age to ours, constructing on a scale that dwarfed everything that had gone before except the colossal buildings of Egypt. Under a new stimulus, and with new means of effect, Roman building greatly outstripped that of the Greeks in extent, in variety, and magnificence.”

An arch built on its side, with its convexity upstream, and its ends braced against rocky banks, serves admirably as a dam. It has in many cases withstood floods much higher than those expected by its designers. Such dams must not be too long, or what is saved in thickness is more than lost in length. Arches inverted are used in many places as gulleys for drainage. Near Bristol, in England, they anchor the cables of the Clifton Suspension Bridge, at a depth of eighty-two feet below the surface of the ground. Many tunnels finished in masonry have outlines which are two arches united, the lower arch being inverted. The Cloaca Maxima, the famous sewer at Rome, is of this pattern; it is twenty-six feet high, sixteen feet broad, and is now in its twenty-fifth century of service.

Ferguson locking-bar pipe. East Jersey Pipe Co,. Paterson, N. J.