in geometry, any part of the circumference of a circle or curved line, lying from one point to ano- ther, by which the quantity of the whole circle or line, or some other thing sought after, may be gathered.

a concave or hollowed piece of building, con- structed in such a manner that the several stones of which it is composed keep one another in their places. The terms arch and vault properly differ only in this, that the arch expresses a narrower, and the vault a broader piece of the same kind. The principal diffe- rence in the form of arches is, that some are circular, and others elliptical; the former having a larger or smaller part of a circle, the other of an ellipse. What are called straight arches, are those frequently used over doors and windows, the upper and under edges of which are straight and parallel, and the ends and joints all point- ing toward a centre. The space between two piers of a bridge is called an arch, because usually arched over.

Triumphal Arches are magnificent entries into ci- ties, erected to adorn a triumph, and perpetuate the me- mory of the action. The arches of Titus and Con- stantine make at this time a great figure among the ruins of old Rome.

in composition, signifies chief, or of the first class; as archangel, archbishop, &c.

ARCHÆUS, or ARCHUS. See ARCHEUS.