Among the first
time-measuring devices were sundials. Early
on, observers noticed that the shadow cast by a post changed in length and
direction as the sun traveled across what they then believed was the vault of
the sky. By 3500 B.C., the
Egyptians had constructed elaborate obelisks whose moving shadows broke the day
into two parts, divided by noon. The
thousand year later, the obelisk had evolved into the smaller sundial, or shadow
clock; for millennia it remained the most advanced device for dividing days into
hours. The sundial, however, had
obvious shortcomings: it didn’t work at night or on cloudy days, and since the
length of the day varied everywhere, sundial time varied everywhere, too.
One
way of getting around the pesky unreliability of the sun was the water
clock-water flowed both day and night (freezing weather was of course a time
stopper) and could be contained in a small vessel.
A half-century after the first sundials, Egyptians kept time by measuring
water as it dripped from a hole in the bottom of a bowl.
A series of lines, etched around the inside of the bowl, indicated the
passage of time. The water clock
required sophisticated calibration, since the water emptied out faster when the
bowl was full and the pressure greater.
For
thousands of years, water served as a backup for the fickle sun.
David Thompson, the curator of horology at London’s British Museum, who
oversees a collection of more than 7,000 timepieces, describes one of the more
elaborate creations. Built for a
Chinese emperor in A.D. 1090 by a civil servant named Su Sung, this “Heavenly
Clockwork” consisted of a 30-foot-high pagoda-like tower.
Inside, a waterfall powered the works.
Outside, a celestial globe rotated at the peak of the structure.
Beneath it, doors opened to reveal five storied of figures who clanged
bells and gongs at appropriate moments to sound out the time.
He
goes on to explain that, what the refinement of glassmaking in eighth-century
Europe, the hourglass, or sandglass, became the standard measure of time.
Though impervious to both clouds and cold, the weight of sand was a
drawback. Therefore, sandglasses
were small and used mainly for short-interval timing: sermons, lectures and,
eventually, determining the speed of a ship.
To
accomplish that, a seaman tossed overboard a rope with knots slipped through his
fingers while another crewman monitored a sandglass that measured 28 seconds.
If five knots passed through the seaman’s hand in that interval, the
ship was moving at five “knots”-aeronautical and aeronautical measure of
speed use to this day. (One knot
equals 1.15 miles per hour.)
timekeeping devices