_) of the principal aqueducts varies
considerably at different points of their course. The Anio Novus has the
largest of them all, measuring 3 to 4 ft. wide and 9 ft. high to the top
of the roof, which is pointed. They are lined with hard cement (_opus
signinum_) containing fragments of broken brick. Those aqueducts of
which the most conspicuous remains exist in the neighbourhood of Rome
are the four from the upper valley of the Anio, the two which took their
supply and their name from the river itself, and the Marcia and the
Claudia, which originated from the same group of springs, in the floor
of the Anio valley 6 m. below Subiaco. Those of the Anio Vetus, which
travelled at a considerably lower level than the other three, are the
least conspicuous, while the Claudia and Anio Novus as a rule kept close
together, the latter at the highest level of all. The ruins of bridges
and substructions in the Anio valley down to Tivoli, though
comparatively little known, are of great importance. In all the
aqueducts the original construction of the bridges was in _opus
quadratum_ (masonry), while the substructions are in brick-faced
concrete; but the bridges are as a rule strengthened (and often several
times) with reinforcing walls of concrete faced with _opus reticulatum_
or brickwork. Below Tivoli, where the Anio leaves its narrow valley, the
aqueducts sweep round towards the Alban hills, and pass through some
very difficult country between Tivoli and Gallicano, alternately
crossing ravines, some of which are as much as 300 ft. deep, and
tunnelling through hills.[3]

The engineering skill displayed is remarkable, and one wonders what
instruments were employed--probably the so-called _chorobates_, an
improvement upon the ordinary water-level (Vitruvius viii. 6), though
this would be slow and complicated. The optical properties of glass
lenses were, however, unknown to the ancients, and the _dioptra_, or
angle measure, was considered by Vitruvius less trustworthy than the
_chorobates_ for the planning of aqueducts (cf. E. Hultsch, _s.v_. in
Pauly-Wissowa, _Real-encyclopadie_). The aqueducts as a rule were
carried on separate bridges, though all four united at the Ponte Lupo, a
huge structure, which after the addition of all the four, and with the
inclusion of all the later strengthening walls that were found necessary
in course of time, measures 105 ft. in height, 508 in length, and 46 in
thickness at the bottom, without includin