The Hourglass, or Sandglass
Two glass bulbs and a trickle of sand: the hourglass never told the time of day, but for centuries it measured intervals at sea, in church and at work.
What an hourglass is
An hourglass is a timer, not a clock. Britannica describes it as two pear-shaped glass bulbs joined at their narrow ends by a very small passage, filled with sand or, occasionally, mercury. The passage is sized so the filling runs from the top bulb to the bottom in a fixed time, whether an hour or a minute, and turning the glass over starts the count again.
Despite the name, glasses were made for many different spans. Museum collections include hour, half-hour and quarter-hour glasses, and log glasses that run for only a few seconds. The general term sandglass covers them all.
How it works and what affects it
The running time depends on two things: how much sand is in the glass and how wide the opening between the bulbs is, as the Antiquarian Horological Society explains. Before glassmakers could form the whole thing in one piece, makers joined two separate bulbs around a thin disc with a hole in its center. The bulbs were sealed with wax, and the joint was wrapped and tightly bound.
Over the centuries makers tried many materials in search of a fine, free-running filling. The main enemy was damp, the Society notes, because moist sand clumps and stops flowing evenly. Even so, Britannica cautions that hourglasses were never especially accurate. Their strength was simplicity: no springs, weights or pendulums, and nothing to wind.
See also: a stopwatch for when there is no sand to hand on OnlineClock.app.
Where and when it began
No one knows who invented the hourglass. Guinness World Records mentions a story crediting an eighth-century French monk named Luitprand, but says there is no evidence for it, and suggests the device was probably developed in the 11th or 12th century by Islamic or European makers. That remains a suggestion rather than a documented fact.
The earliest firm evidence is a painting. The Antiquarian Horological Society points to The Allegory of Good Government, a fresco by Ambrogio Lorenzetti in Siena dated 1337 to 1339, in which one figure holds a sandglass; Guinness gives the year as 1338. Guinness also notes that the figure represents temperance, which suggests the object was already familiar enough to stand for a virtue, and that sandglasses turn up in European ship inventories later in the 14th century.
Whatever its exact origins, the hourglass never told the time of day. Britannica describes it as a device for measuring intervals: it marks off a fixed span, and someone has to be there to turn it over when the sand runs out.
See also: the long road from sandglass to atomic clock on OnlineClock.app.
Sandglasses at sea
Ships were the hourglass's natural home, because unlike a pendulum, a trickle of sand is not upset by a rolling deck. Royal Museums Greenwich holds several working ship's glasses. One, from the 17th century, is a 60-minute glass filled with yellow sand, its central joint bound in leather. Its wooden stand, now broken into five fragments, once had circular bases and four rounded carved supports plus a square one. Another, from the 18th century, runs for 30 minutes on white sand and sits in a cylindrical brass stand with five straight supports, with a brass wire becket at each end to hang it up by. Both belong to the Caird Collection at the National Maritime Museum.
Very short glasses had a special job. Britannica notes that a sandglass used with the common log to estimate a ship's speed was called a log glass. The museum's 19th-century example is a small glass of the usual hourglass shape held inside a brass cylinder, with two openings through which the glass and sand can be watched, and it is a 14-second glass.
In church and at home
On land, Britannica says, hourglasses were once common in churches. The British Museum holds a German multiple glass of about 1750 of a kind often called a pulpit timer, because preachers used such sets to time their sermons, according to the Antiquarian Horological Society.
Multiple glasses could be elaborate. Another British Museum piece, from the late 17th century, mounts four glasses in an ebony frame, running for a quarter hour, half an hour, three quarters and a full hour, with a leather case. A story links it to Mary, Queen of Scots, and Francis II of France, but the Society explains why that is doubtful: the Tudor arms painted on it are from the wrong period, one lion faces the wrong way and the monogram on the case was added later.
How to recognize an old sandglass
Clues to age are the ones the museums record. Older glasses are often made from two separate bulbs, with the joint between them bound in leather, twine or similar wrapping, and the glass may have a greenish tint. Stands are typically wood or brass, with posts or supports between the top and bottom plates, and some have loops for hanging. The surest way to learn a glass's running time is to time it, keeping in mind that damp sand runs unevenly.
Questions people ask
Is an hourglass the same as a sandglass?
Yes. Sandglass is the general term, and hourglass is the everyday name, even for glasses that run for much less than an hour. A glass used at sea to help measure a ship's speed was called a log glass.
When was the hourglass invented?
Nobody knows. The earliest firm evidence is Ambrogio Lorenzetti's fresco in Siena, painted between 1337 and 1339, in which one figure holds a sandglass.
Why were hourglasses used on ships?
The motion of a ship does not disturb falling sand the way it disturbs a pendulum, so sandglasses could time intervals at sea. Short log glasses were also used with the common log to estimate a ship's speed.