vaults, while, at Nimroud,
channels were discovered which were square in section and covered with
large slabs of limestone.[291] The Assyrian architects seem, however, to
have had a decided preference for the vault in such a situation. They
expected it to give greater solidity, and in that they were not mistaken.
The vaults of burnt brick, though set without cement, have remained
unshaken and close in their joints, and the sewers they inclose are the
only voids that have remained clear in the ruins of the buildings to which
they belong.

[Illustration: FIG. 94.--Sewer at Khorsabad, with elliptical vault;
compiled from Place.]

We may, perhaps, be accused of dwelling too minutely upon these Assyrian
vaults. We have done so because there is no question more interesting or
more novel in the whole history of architecture than the true origin of the
keyed vault and the different uses to which it has been put. Ottfried
Mueller looked upon the Etruscans as the inventors of the vault; he believed
that the Greek builders learnt the secret from the early inhabitants of
Italy,[292] and that the arches of the Roman _Cloaca Maxima_ built by the
Tuscan architects of the Tarquins, were the oldest that had come down to
us from antiquity. The archaeological discoveries of the last fifty years
have singularly falsified his opinion and given an age to the vault never
before suspected. Even in the days of the Ancient Empire the Egyptians seem
to have understood its principle; in any case the architects of Amenophis,
of Thothmes, of Rameses, made frequent and skilful use of it long before
the Ninevite palaces in which we have found it were erected.[293] But the
possession of stones of enormous size enabled the Egyptians to dispense to
a great extent with the arch, and we need not be surprised, therefore, that
they failed to give it anything like its full development. They kept it in
the background, and while using it when necessary in their tombs, in the
outbuildings of their temples, in their private dwellings and warehouses,
they never made it a conspicuous element of their architectural system.
They may well be admired for the majesty of their colonnades and the
magnificence of their hypostyle halls, but not for the construction of
their vaults, for the imitation of which, moreover, they gave little
opportunity.

In Chaldaea and Assyria the conditions were different. Supposing the
architecture of those two countries to be yet entire, s