Timekeeping  /  The sand glass
1338

The sand glass

Sand through a waist of glass. First seen in a Siena fresco; by 1345 ships bought them by the dozen.

The sand glass
Temperance holding a sand glass, in Ambrogio Lorenzetti’s Allegory of Good Government, Siena, 1338–39. It is the earliest known picture of the instrument.Photograph: Palazzo Pubblico, Siena · public domain painting, photograph via Wikimedia Commons (The Yorck Project)
Two bulbs joined at a narrow neck. Dry sand runs through at a steady rate however much is left, so one turn measures a fixed span.Drawing: Watch Historian
The need
Water clocks froze, spilled and clogged. A sealed glass of dry sand measured a short, repeatable interval anywhere: in a pulpit, a kitchen, or a ship at sea.
How it worked
Two glass bulbs are joined at a narrow waist and sealed. Fine sand, or powdered marble or eggshell, runs through at a steady rate regardless of how much remains, so one turn marks a fixed span such as half an hour.
What it changed
Timing at sea became possible: the glass set the crew’s watches and, with a log line, gave the ship’s speed. A popular story credits an eighth-century monk, Liutprand of Chartres, with the invention; it first appears in a magazine article of 1869 and has no medieval source. The real evidence begins in the early fourteenth century: a ship’s “arlogio” mentioned about 1313, this fresco, and in 1345–46 twelve “glass clocks” bought at Sluys for Edward III’s ship La George.
Where it is now
Sala dei Nove, Palazzo Pubblico, Siena.

SourcesBalmer, “The Operation of Sand Clocks and Their Medieval Development”, Technology and Culture 19 (1978) · Turner, “The Accomplishment of Many Years”, Annals of Science 39 (1982) · Dohrn-van Rossum, History of the Hour (1996), pp. 117–118 · Nicolas, A History of the Royal Navy II (1847), p. 476

Suggest a correction

Spotted an error, or know a better source? Tell us. Notes go to the editor and are not published.