An hourglass is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated flow of a substance from the upper bulb to the lower one due to gravity. Typically, the upper and lower bulbs are symmetric so that the hourglass will measure the same duration regardless of orientation. The specific duration of time a given hourglass measures is determined by factors including the quantity and coarseness of the particulate matter, the bulb size, and the neck width.
German half-hour sand glass, first quarter of the 16th century, bronze-gilt and silver-gilt, height: 8.3 cm, diameter: 8.4 cm, Metropolitan Museum of Art (New York City)
A winged hourglass as a literal depiction of the Latin phrase tempus fugit ("time flies")
Sarcophagus dated c. 350, representing the wedding of Peleus and Thetis (observe the magnification with the object held by Morpheus in his hands)
Temperance bearing an hourglass; detail Lorenzetti's Allegory of Good Government, 1338
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.
The flow of sand in an hourglass can be used to measure the passage of time. It also concretely represents the present as being between the past and the future.
Horizontal sundial in Canberra
24-hour clock face in Florence
A contemporary quartz watch, 2007
