Ancient Rome received all of its water (according to Encarta,
about 38 million gallons a day) through a system of aqueducts.
All water flowed to the city by gravity, but because it was
arriving from surrounding hills, it could be stored in large
cisterns very similar in concept to today's water towers (the
main difference is that cisterns are filled from the top). Water
flowed from the cisterns either through pipes to individual
houses or to public distribution points. Fountains served both
decorative and functional purposes, since people could bring
their buckets to the fountain to collect water. The cisterns
provided the height needed to generate water pressure for the
fountains to spray. As discussed in How Water Towers Work, a
foot of height generates 0.43 pounds per square inch (psi) of
water pressure, so a cistern does not have to be that tall to
develop enough pressure to give a fountain a reasonable display.