graving No. 1 represents the arch referred to in the description
of the Monjas at Uxmal; and as the stones are not quite horizontal, but
stand nearly at right angles to the line of the arch, it shows how near
an approach was made to the real principle on which the arch is
constructed.

[Engraving 51: Triangular Arch]
Throughout every part of Central America, Chiapas, and Yucatan, the
same method is to be traced with slight modifications. The stones
forming the side walls are made to overlap each other until the walls
almost meet above, and then the narrow ceilings are covered with a
layer of flat stones. In every case the stones were laid in horizontal
layers, the principle of constructing arches, as understood by us,
being unknown to the aboriginal builders. This readily accounts for the
extreme narrowness of all their rooms, the widest not exceeding twenty
feet, and the width more frequently being only from six to ten feet. In
a few cases the covering stone is wanting, and the two sides meet so as
to form a sharp angle. At Palenque the builders did not cut the edges
of the stones, so as to form an even surface, their practice differing
in this respect from that adopted in Yucatan, where in every instance
the sides of the arch are made perfectly straight, or have a slight
curve, with the inner surfaces smooth.

It may now be interesting to inquire if any similarity exists between
the American method and those observed among the nations of antiquity
in Europe and Asia. A true arch is formed of a series of wedge-like
stones or of bricks, supporting each other, and all bound firmly
together by the pressure of the centre one upon them, which latter is
therefore distinguished by the name of keystone.

It would seem that the arch, as thus defined, and as used by the
Romans, was not known to the Greeks in the early periods of their
history, otherwise a language so copious as theirs, and of such ready
application, would not have wanted a name properly Greek by which to
distinguish it. The use of both arches and vaults appears, however, to
have existed in Greece previous to the Roman conquest, though not to
have been in general practice. And the former made use of a
contrivance, even before the Trojan war, by which they were enabled to
gain all the advantages of our archway in making corridors or hollow
galleries, and which, in appearance, resembled the pointed arch, such
as is now termed Gothic. This was effected by cutting