The hourglass, also known as the sandglass, is a timekeeping instrument that has been used for centuries. The sandglass is constructed using two conic reservoirs that are joined at their apexes by a small hole. One reservoir is filled with a finite volume of sand that flows into the bottom reservoir over a set duration of time. When the sand has run completely from the top reservoir through the small opening, the previously determined time period can be recorded and the timepiece inverted to begin running again. The hourglass has been made in various time denominations ranging from 30 seconds to longer than an hour.
The actual date of the hourglass’s emergence cannot be definitely placed. There are, however, many theories as to when the hourglass emerged as a common timekeeping instrument. The most significant clues to the invention of the hourglass deal with the dates of portrayals and the emergence of the technology needed to create the glass to make the reservoirs. No record exists of the sandglass’s original form, so researchers have to work with the modern hourglass shape. Items were made from glass as far back as 2500 BCE, but the art of glass-blowing was not perfected until around 70 BCE.
The earliest definite depiction of an hourglass dates to between 1337 and 1339 CE. This depiction can be found in an Italian fresco on the walls of Palazzo Pubblico in Siena and was painted by Ambrogio Lorenzetti. Another early depiction of an hourglass can be found at the Mattei Palace in Rome. The palace contains a Greek bas-relief depicting Morpheus, the god of time, holding an hourglass. German artist Albrecht Dürer (1471-1528) profiles the hourglass prominently in many of his artistic works. The sandglass holds a prominent place in all three of his most famous copper engravings. Knight, Death, and the Devil (1513), St. Jerome in His Study (1514), and Melancholia I (1514) depict the hourglass just to the left of the central character. The first textual reference can be found in the Receipt of Thomas de Statesham, which dates to around 1345. This receipt references Thomas’s acquisition of 12 hourglasses (“pro xii orologiis vit- reis”) in Flanders. The second textual reference dates to slightly after this, in 1380, and can be found in a furniture inventory of King Charles V of France: “ung grant orloge de mer, de deux, grans fiolles plains de sablon en ung grant estuy de boys garny d’archal” (a large sea clock with two large phials filled with sand, in a large wooden brass-bound case). The hourglass also finds reference in classical literature, specifically in the works of William Shakespeare. Specifically, Henry V (Act 1 Scene I) refers to an hourglass in the opening Chorus.
One theory credits the invention of the hourglass to an 8th-century monk of Chartres named Luitprand, but most historians believe that if there is any truth to this theory it would find credence as a reinvention or an improvement on the ancient sandglass after the Dark Ages. Many historians believe the hourglass came into use about the same time as, or slightly after, the clepsydra (water clock). Since both timekeepers use the same basic principle, emergence during the same approximate time period could be likely. For a long time no distinctions were made between the two, and they were simply called horlogues, leaving only context to determine which of the two was being described. This theory places the invention of the hourglass at around the 3rd century BCE in the Egyptian city of Alexandria. The sand for the hourglass would have been more accessible in regions with drier climates. In countries such as Egypt, water would have been scarce and not a resource that could be needlessly wasted. The more probable date of emergence coincides with the inception of sea travel, as the technology would aid in nautical navigation.
The hourglass has many advantages over the clepsydra. Sand, unlike water, flows at a consistent pace when pressure is applied from above. For this reason, sandglasses kept a more consistent and steady time. Moreover, unlike water, the sand would not freeze or evaporate, so no replacement filler was ever needed. Most high-quality sandglasses did not actually use sand, however, but finely crushed eggshells. The eggshells provided a smoother flow than sand and therefore provided a more accurate method of keeping time.
The hourglass has had many different variations since its invention. Although the hourglass and sandglass have become synonymous in modern meaning, ancient variations of the hourglass used different fillers as the timepiece evolved. Some ancient variations used mercury instead of sand. These variations required an extremely fine hole between the reservoirs to keep the mercury from flowing too quickly. Another mercury variation used a small bubble of mercury inside a narrow tube containing air. The resistance from the internal air pressure allowed the mercury to fall slowly through the tube and therefore allowed the measurement of a useful time period.
Since the original form of the hourglass is not known, it would be fair to consider the clepsydra and early sand clock (clepsammia) among the very early forms of the hourglass. Both of these early timepieces use a set amount of water or sand, respectively, and measured time by the volume of water and sand in a bottom reservoir, but could not be reset without refilling the top.
Ancient and Modern Uses
The hourglass found use in various places throughout history. The most notable of these is in the field of nautical navigation. In the early era of sea travel, it was crucial for ancient mariners to know the speed of their ship in order to know their location at sea. The hourglass provided a timepiece that would remain stable and reliable even in turbulent weather. Thirty-second sandglasses were used to calculate speed. Tying a rope to a heavy piece of wood and throwing the wood overboard behind the ship calculated speed. The wood created resistance with the water and dragged the rope out behind the boat. The rope would have knots tied 47-foot, 4-inch intervals so that each knot that ran through the fingers of the navigator in the 30-second time period indicated one nautical mile in an hour.
Sandglasses were also used as timers for all sorts of activities. They became the first industrial timers for early factories. Das Feuerwerkpuck (1450), a German essay on manufacturing, contains an illustration of a stamping mill being timed by an hourglass. Both politicians and clergy also used the sandglass as speech timers. The hourglass was introduced into the church during the 16th century as a tool to restrain preachers from speaking too long. The hourglass also served as a symbol during olden funerals that the sands of life had run out. In today’s era, the sandglass has all but disappeared as a timer except for use in kitchens as the common 3-minute-egg timer.
Matthew A. Heselton
See also Archaeology; Clocks, Atomic; Clocks, Mechanical; Sundials; Time, Measurements of; Timepieces
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