water over hundreds of miles of channels without running
into the ground before it arrived at its intended destination.
In addition to the aqueducts, concrete also enabled the
construction of an unprecedented series of roads and high-
ways throughout the Roman Empire. Th e Romans built ap-
proximately 53,000 miles of roads; by comparison, the U.S.
Interstate Highway System had a total of about 47,000 miles
at the beginning of the 21st century. Romans invented the
layered method of building roads, starting with large stones
at the bottom to absorb moisture and prevent shift s in the
ground and ending with smooth stones held together with
concrete. Many of the roads constructed during the Roman
Empire are still in use today, such as the Via Appia (the Ap-
pian Way), which was begun in 312 b.c.e. Roman roads are
found throughout Europe, Asia, and Africa, where many of
the paths laid out before the birth of Christ have been paved
over with modern materials and are still used.
Inside the home Romans are credited with the invention
of the hypocaust heating system, a way of moving heated air
through houses to warm them. Th is method, which is the ba-
sic design for modern-day heating and air conditioning sys-
tems, consisted of raising houses a few feet above the ground
and leaving space in the walls for air fl ow. Vents in the walls
would pull warm air from a small fi re under the house, dis-
tributing it throughout the entire building. Because a large
fi re would create smoke the fi re had to be small, but it had
to be constantly going or it would lose its eff ectiveness. Th e
Romans relied on slaves to tend to the house fi res throughout
the day.
Many of the inventions credited to Rome are actually
variations on designs developed by other civilizations, par-
ticularly the Greek culture that preceded Rome in history.
Oft en adaptations were just the result of the natural process
of improving on what works, though in some cases there were
changes signifi cant enough to consider the Roman version a
new entity. Th is is true of the Roman version of the water-
wheel, which captures the power of fl owing water to move a
heavy stone to grind grain into fl our, credited to Marcus Vit-
ruvius, an architect and engineer who served under Augustus
in the fi rst century b.c.e. His wheel improved on a similar
concept from the Greeks: While the Greek wheel was moved
horizontally as water fl owed by, the Roman waterwheel stood
horizontally and had water stream down onto it. Gravity gave
the falling water much more force than its natural fl ow pro-602 inventions: RomeRoman aqueduct in Tunis; Roman improvements in concrete enabled them to build hundreds of miles of water-transportation systems, considered
one of the greatest achievements of the Roman Empire. (© Board of Regents of the University of Wisconsin System)