Hourglass

An hourglass (or sandglass, sand timer, or sand clock) 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 (historically sand) from the upper bulb to the lower one by 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. Depictions of an hourglass as a symbol of the passage of time are found in art, especially on tombstones or other monuments, from antiquity to the present day. The form of a winged hourglass has been used as a literal depiction of the Latin phrase tempus fugit ("time flies"). The origin of the hourglass is unclear. Its predecessor the clepsydra, or water clock, is known to have existed in Babylon and Egypt as early as the 16th century BCE. There are no records of the hourglass existing in Europe prior to the Late Middle Ages; the first documented example dates from the 14th century, a depiction in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti. Use of the marine sandglass has been recorded since the 14th century. The written records about it were mostly from logbooks of European ships. In the same period it appears in other records and lists of ships stores. The earliest recorded reference that can be said with certainty to refer to a marine sandglass dates from c. 1345, in a receipt of Thomas de Stetesham, clerk of the King's ship La George, in the reign of Edward III of England; translated from the Latin, the receipt says: in 1345: The same Thomas accounts to have paid at Lescluse, in Flanders, for twelve glass horologes (" pro xii. orlogiis vitreis "), price of each 41⁄2 gross', in sterling 9s. Item, For four horologes of the same sort (" de eadem secta "), bought there, price of each five gross', making in sterling 3s.