The ancient art of telling time with fire and smoke

In China, the use of incense smoke to tell time dates back to the 6th century CE. Incense would be lit at the bottom of a maze and burn its way through, with outlets to vent the smoke and indicate the time.

Different scents may have even been used along the maze to tell the current time by a whiff. And there were even incense alarm clocks where the incense would burn through strings and drop metal balls into a bowl to sound the alarm.

Incense timers were used in China into the 21st century by coal miners to guage the length of time spent underground.

From JSTOR Daily:

According to historian Andrew B. Liu, incense had been used to measure time since at least the sixth century, when the poet Yu Jianwu wrote:

By burning incense [we] know the o'clock of the night,
With graduated candle [we] confirm the tally of the watch.

The incense clock takes the basic concept—timing by combustion—and elevates it to a new level of gorgeous complexity. Examining the example held by the Science Museum, I was struck by its diminutive size: no larger than a coffee mug. Yet its small compartments are carefully packed with everything it needs to operate. In the bottom tray, you'll find a bite-sized shovel and damper; above that, a pan of wood ashes for laying out the incense trail; then, stacked on top, an array of stencils for laying out the labyrinths. As Silvio Bedini, historian of scientific instruments, explains in his extensive study of the use of fire and incense for time measurement in China and Japan, the variety allows for seasonal variation: longer paths to be burned through the endless winter nights, while shorter ones serve for summer.

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