The water-clock or clepsydra had been used for thousands of years by the Nilotic priest astronomers to measure the passage of time. The word clepsydra is a compound of the Greek words kleptein, "to steal" and hydor, "water."
An ancient Egyptian water-clock dates to the reign of Amenhotep III (BC 1417-1379). It was used in the Temple of Amun-Ra at Karnak. The bowl-shaped outflow is the simplest form of a water-clock and is known to have existed in Babylon and in Egypt as early as 1600 BC.
The clepsydra was used by the Chinese, the Babylonians, the Greeks, and the Romans. Early clepsydras were brass bowls with a small hole at the bottom. The bowl was placed inside a larger bowl of water which flowed through the opening at a rate dependent upon the size of the hole. When the bowl sank, that marked the end of the time to be measured.
Water-clocks often had marks of the sun's motion on the first container. As water dripped from it into another basin, the drop in water level showed the passage of up to eight hours. The second container was not always used. Some water-clocks allowed the water to drip on the ground.
In ancient Greece, the clepsydra was used to limit the time a lawyer or judge could speak in court. In Philostratus's Life of Apollonius of Tyana we read: "And how long will your pleading last by the water-clock's reckoning?" (Part IV)
According to Aeschines, the political opponent of Demosthenes, "The first [clepsydra] water was given to the accuser, the second to the accused, and the third to the judges." The guardian of the clepsydra stopped the flow of water during the reading of documents in evidence so that the reading time was not charged to the speaker.
The ancient clepsydra remained the most accurate clock ever constructed until the Dutch physicist Christiaan Huygens built a working prototype of a pendulum clock in 1656.
The clepsammia or sand-glass is an interval timer that depends on the particle flow of sand. Its older counterpart was the clepsydra which measured by the flow of water. According to the American Institute of New York, the clepsammia was invented at Alexandria about 150 BC. However, the Greeks were using the sand clock as early as 300 BC.
The sand-glass appears to have come into wide use after much of the ancient world became drier. Earlier, the clepsydra or water-clock was much more common, but there were difficulties with the water flow in the colder climates. Caesar was much aggrieved when he invaded Gaul and found that the water in his clepsydra had frozen.
There are difficulties with the sand-glass also. If moisture penetrated the seal on the glass, the sand clumps. To prevent this, powdered eggshell or lead dust were sometimes added. The problem of moisture was largely overcome by the nineteenth century when glassblowers fused the two bulbs together at the joint, making them airtight.
Sand-glasses generally were used to measure an hour. They were called horologes, from the Greek words hōra, "hour" and legein, "to gather."
In ancient philosophy the term horos referred to the boundaries of an area, or a landmark, or a term. It is likely a reference to Horus who was said to be the fixer of time, current, tides, winds and boundaries. The English words hour, horizon and horoscope share this root. Today the word horoscope connotes astrology, but the original meaning was "observer" [skopos] of the hour. The Indo-European root for year is yeHr-, another reference to Horus. The association of Horus with the horizon is seen in the word Har-ma-khet, meaning "Horus of the Horizon."
The sand-filled hourglass became popular as a personal time device for European nobility in the thirteenth century. Charlemagne (742-814) possessed a 12-hour sand-glass. These timers were used in the kitchens of wealthy households to judge cooking times. Thomas de Stetsham, a ship clerk in the service of King Edward III (1312-1377) ordered 16 sand-filled hourglasses. The sand-glass was used to time the length of the watches on ships.
The hourglass served to time the length of sermons in churches. The device installed in the Royal Chapel in London measured a period of eighteen minutes, the time ordered by Queen Victoria, who apparently did not appreciate long-winded clerics.
Doubtless the hourglass resting near the pulpit served as a congregational reminder of the passage of time and our mortality. Wisdom begins with this humbling truth, as Psalm 90:12 reminds us: "Teach us to number our days, that we may gain a heart of wisdom."
Related reading: Theories of Time