HEMICYCLUM, compounded of ἡμίς, 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 may be no joint in the middle, where the key-stone should be.

Hemicyclium was also a part of the orchestra in the ancient theatre. Scaliger, however, observes that it was no standing part of the orchestra, being only used in dramatic pieces, where some persons are supposed to have 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 of which looked to the north. There was a style or gnomon issuing from the middle of the hemicycle, of which that point corresponding to the centre of the hemicycle represented the centre of the earth; and its shadow was 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, and so forth.