as-relief (see ARCHITECTURE, fig. 10), brought by Layard from
Kuyunjik, are representations of semicircular and ovoid domes, which
show that the feature was well known in Assyria, and as they build domes
of the same nature down to the present day and without centring of any
kind, it suggests that they may have existed from the remotest ages. The
most ancient examples in Europe are those of the "beehive" tombs at
Mycenae and elsewhere in Greece, ascribed generally to the 11th century
B.C. In a sense, they are not true domes, because they are built in
horizontal courses of stone, which act like the voussoirs of an arch in
resisting the thrust of the earth at the back. This did not exist in the
Choragic Monument of Lysicrates or other circular buildings in Greece,
because their vertical sections were not portions of circles. For this
reason, the conical vault of the Baths in Pompeii is not a dome. The
circular Laconicon in the Baths of Titus (A.D. 72) may have been domed,
and the great hemicycles in the Thermae must certainly have been roofed
with semi-domes.

The earliest Roman domes are those of the great circular halls at Baiae
near Naples, described as temples, but really forming part of the
immense bathing establishments there, the favourite place of resort of
the Romans during the latter part of the Republic. The largest on the
east side of the Lake of Avernus, known as the Temple of Apollo, is a
circular hall with an internal diameter of 100 ft. Those of Diana,
Mercury and Venus at Baiae, were 96, 66 and 60 ft. respectively. The
vaults were all built in tufa with horizontal courses in brick and
cement. Half of the dome of the Temple of Mercury had fallen down,
showing the section to have been nearly that of an equilateral arch.
From the fact that there were pierced openings or windows in all these
domes, they probably constituted the _frigidaria_ of the baths.

The first example still existing in Rome is that of the Pantheon (A.D.
112), where a circular dome, 142 ft. in diameter, rests on a circular
wall, its height being about equal to its diameter. The lower courses of
this dome, built in the Roman brick or tile, were, up to the top of the
third coffer, all laid in horizontal courses; above that, the
construction is not known for certain; externally a series of small
arches is shown, but they rested on a shell already built. The so-called
Temple of Minerva Medica (now recognized as the Nymphaeum of the Baths
of G