Hourglass
The simplest clock turned out to have a catch. Water runs out more slowly the less of it there is, but sand pours at the same rate down to the last grain. Physicists only explained why in the 20th century.
- 1338the first known picture, a fresco in Siena
- 18 glassescarried by Magellan's fleet
- 28 secondsthe glass for the log that measured knots
Younger than sundials and water clocks
Hourglasses look ancient, but reliable evidence for them only appears in the 14th century. The first known picture is Ambrogio Lorenzetti's fresco of Good Government, painted in 1338 in Siena's town hall.
On the right is a glass in a turned frame. The sand flows in real time, and the readout counts what is left.
A stream that doesn't care
In a water clock the whole column presses on the hole, and the jet weakens as the level falls. Sand behaves differently. The grains lean on each other and on the walls, forming arches, and the weight of the upper layers goes into the glass rather than the neck. This is called the Janssen effect.
So the level above sinks evenly and the glass needs no marks: it simply measures the same time, every time.
Width is everything
In 1961 the Dutch engineer Beverloo found the formula: the flow of sand grows as the neck width to the power of five halves. Halve the neck and the glass runs almost six times longer.
W = 0.58 · ρ · √g · (D − 1.5d)^5/2We have narrowed the neck to 1.4 mm. Move the pointer or swipe sideways over the drawing to change it yourself, and watch the stream thin out or thicken.
Marble dust and eggshell
Ordinary river sand is poor for a glass: its grains are uneven and sharp and wear the neck. Makers used ground marble, crushed eggshell, tin or lead oxide. The powder was dried and sifted until every grain was the same.
Until the 18th century the two bulbs were blown separately, joined through a brass washer with a hole, and the seam was bound with thread and wax.
Time at sea
At sea the sand glass was king. The ship's boy turned the half-hour glass and struck the bell: one stroke, two, and so on up to eight, when the four-hour watch ended. That is where "eight bells" comes from.
We have just turned the glass over, as the ship's boy would. The "Turn over" button on the drawing does the same.
The log and 28 seconds
A ship's speed was measured with a log: a board on a line knotted every 47 feet 3 inches went over the side, and a small 28-second glass was turned. However many knots ran out before the glass emptied, that many nautical miles an hour the ship was making. Speed at sea is still measured in knots.
In the pulpit and the kitchen
In churches a sand glass stood on the pulpit, so the congregation could see how long the sermon had left to run. On gravestones the winged hourglass became the sign that time is passing. And in the kitchen its little sister, the three-minute timer, still measures a soft-boiled egg.
An hourglass has no gears, no springs and no battery. Frost, rolling seas and damp don't upset it, as long as the sand stays dry. Perhaps that is why it outlived both the water clock and the pendulum.