No one in Egypt, and no one around the fertile crescent of Mesopotamia, had ever seen the Sun shine at night. The people could obtain a fair idea of time by the stars; but the remote heavenly bodies need careful observation and calculation if they are to give time truly. A man, wanting to know the approximate time of night, could not go out and glance at a shadow cast by a star on a dial. He had to perform a much more elaborate ritual. To fill the need for telling time casually and quickly at night, the Egyptians invented water clocks.
Almost simultaneously two kinds of water clocks were invented. The outflow clocks had to be refilled every evening at sunset. The water flowed out of a hole near the bottom. At sunrise they were empty. On the inside were marked twelve divisions, like the divisions in a measuring cup, and a man need only observe the level of the water to determine the correct time.
The principle was very simple, but the Egyptians expended a great deal of ingenuity and scientific care in their construction. Water flows swiftly under pressure, but as the pressure is decreased, the rate of its flow lessens. So in the evening, when the water clocks were newly filled, the water level would sink very fast; but toward morning, when most of the water had flowed out, the change would be quite slow.
To remedy this defect some of the clocks were made in the shape of a parabolic curve, wider at the top than at the bottom, and provided with tapering cross sections. With this improvement the rate of fall of the water line remained steady, even though the head of water fell and the rate of flow diminished. In this way, equal graduations could be used.
Inflow clocks gauged the change in time by a rising water level. Above the clock itself was a vessel which continually overflowed so that the head of water could be kept constant. In the bottom of the auxiliary vessel a tiny hole was pierced, and water dripping from it filled the water clock. Some of these inflow clocks are marked with sloping hour line. On one side you might look for the “unequal hours” in summer, and on the opposite side for them in winter.
Aside from all practical uses, these clocks served as part of the ornamental furniture of a noble Egyptian household. Curious animals, called cynocephali, protruded monkey bodies and dog faces from the front of the vessels. Tiny holes led some of the water into little basins, too small to serve any purpose but decoration, and frequently there were steps to give the effect of a waterfall when the clock was drained in the morning.
A long time after these clocks were invented, some roving Greek merchant saw one, and promptly took the idea home to his country. Never for a moment did he think of using the clock to tell the time at night; a much more immediate Greek need inspired him. With a clock, he could at last make some limit to the length of public speeches. The Greek orators were long-winded people, and the populace welcomed the timepiece with open arms. Water clocks were set up in public places where the politicians harangued. Water docks were established in the law courts. The length of speech allowed, depended upon the seriousness of the trial. More time was given to the defendant in a murder case than to a plea involving simple property or inheritance rights.
Anyone who has ever listened to a man spouting forth his declaration of innocence can have sympathy with the use to which the Greeks first put their clocks. Of course, quite early they were placed in private houses, particularly among the wealthier citizens of Athens; but apparently no one before Plato used them for telling time at night.
In 135 B. C. a mathematician of Alexandria invented a water clock which for ingenuity might well rival some of the elaborate timekeepers to be seen, complete with moving figures, on the Wells and Dijon cathedrals. This early example of decorative timekeeping was equipped with wheels which turned by an even drip of water, and raised upward the small statue of a man. In his hand he held a stick, and with it he pointed to the hours marked on a little column at his side. For Greek art, this little clock was unusually ornate, but thoroughly charming, and it indicates how far the knowledge of mechanics had progressed by the second century B. C.
While the science of water clocks was reaching such a high state of perfection, the Greeks were experimenting with other mediums. In the third century B. C. they evolved a more permanent and portable clock which was filled with sand. For this type, too, the vessels must be curved, if the level is to fall with an equal rate of speed; but, whereas the water clocks were single, and needed constantly to be refilled, the sand glass was double. When the sand had run through one side the clock was turned upside down and, by the natural influence of gravity, its flow commenced anew.