16. Flat arch, where the soffit is horizontal and sometimes slightly cambered (dotted line).
17. Upright elliptical arch, sometimes called the egg-shaped arch, employed in Egyptian and Sassanian architecture.
18. The Tuscan arch, where the extrados takes the form of a pointed arch.
19. The joggled arch used in medieval chimneypieces and in Mahommedan architecture.
20. The discharging or relieving arch, built above the architrave or lintel to take off the weight of the superstructure.
21. The relieving arch as used in Egypt, in the pyramid of Cheops; and in Saxon architecture, where it was built with Roman bricks or tiles, or consisted of two sloping slabs of stone.
(R. P. S.)
[1] The ultimate derivation of “arch” is the Latin arcus, a bow, or arch, in origin meaning something bent, from which through the French is also derived “arc,” a curve. In French there are two words arche, one meaning a chest or coffer, from Latin arca (arcere, to keep close), hence the English “ark”; the other meaning a vaulted arch, such as that of a bridge, and derived from a Low Latin corruption of arcus, into arca (du Cange, Glossarium, s.v.). The word “arch,” prefixed to names of offices, seen in “archbishop,” “archdeacon,” “archduke,” &c., means “principal” or “chief,” and comes from the Greek prefix ἀρχ- or ἀρχι- from ἄρχειν, to begin, lead, or rule; it is also prefixed to other words, and usually with words implying hatred or detestation, such as “arch-fiend”, “arch-scoundrel”; it is from an adaptation of this use, as seen in such expressions as “arch-rogue,” extended to “arch-look,” “arch-face,” that the word comes to mean a mischievous, roguish expression of face or demeanour.