from which it gradually escaped through fine perforations at the base". The clock in Apuleius' account was one of several types of water clock used. Another consisted of a bowl with a hole in its centre, which was floated on water. Time was kept by observing how long the bowl took to fill with water.
Although clepsydrae were more useful than sundials—they could be used indoors, during the night, and also when the sky was cloudy—they were not as accurate; the Greeks, therefore, sought a way to improve their water clocks. Although still not as accurate as sundials, Greek water clocks became more accurate around 325 BC, and they were adapted to have a face with an hour hand, making the reading of the clock more precise and convenient. One of the more common problems in most types of clepsydrae was caused by water pressure: when the container holding the water was full, the increased pressure caused the water to flow more rapidly. This problem was addressed by Greek and Roman horologists beginning in 100 BC, and improvements continued to be made in the following centuries. To counteract the increased water flow, the clock's water containers—usually bowls or jugs—were given a conical shape; positioned with the wide end up, a greater amount of water had to flow out in order to drop the same distance as when the water was lower in the cone. Along with this improvement, clocks were constructed more elegantly in this period, with hours marked by gongs, doors opening to miniature figurines, bells, or moving mechanisms. There were some remaining problems, however, which were never solved, such as the effect of temperature. Water flows more slowly when cold, or may even freeze.
Although the Greeks and Romans did
much to advance water clock technology,
they still continued to use shadow clocks.
The mathematician and astronomer
Theodosius of Bithynia, for example, is said
to have invented a universal sundial that
was accurate anywhere on Earth, though
little is known about it. Others wrote of the
sundial in the mathematics and literature
of the period. Marcus Vitruvius Pollio, the
Roman author of De Architectura, wrote
on the mathematics of gnomons, or sundial blades. During the reign of Emperor Augustus, the Romans constructed the largest sundial ever built, the Solarium Augusti. Its gnomon was an obelisk from Heliopolis. Similarly, the obelisk from Campus Martius was used as the gnomon for Augustus' zodiacal sundial. Pliny the Elder records that the first sundial in Rome arrived in 264 BC, looted from Catania, Sicily; according to him, it gave the incorrect time until the markings and angle appropriate for Rome's latitude were used—a century later.
500 BC – 1 BC