Hemicyclium, compounded of ἡμίσημος, half, and κύκλος, circle; a semicircle.

Hemicycle is particularly applied, in Architecture, to vaults in the cradle form; and arches or sweeps of vaults, constituting a perfect semicircle. To construct an arch of hewn stone, they divide the hemicycle into so many voussoirs; taking care to make them an uneven number, that there be no joint in the middle, where the key-stone should be. See Key and Bridge. HEMICYCLIUM was also a part of the orchestra in the ancient theatre. Scaliger, however, observes, it was no standing part of the orchestra; being only used in dramatic pieces, where some person was supposed to be arrived from sea, as in Plautus's Rudens.

The ancients had also a sort of sun-dial, called hemicyclium. It was a concave semicircle, the upper end or cusp whereof looked to the north. There was a style, or gnomon, issuing from the middle of the hemicycle, whereof that point corresponding to the centre of the hemicycle represented the centre of the earth; and its shadow projected on the concavity of the hemicycle, which represented the space between one tropic and another, the sun's declination, the day of the month, hour of the day, &c.