In China, the use of incense smoke to tell the time dates back to the 6th century CE. Incense would be lit at the bottom of a maze and burn through it, with outlets to vent smoke and tell the time.
Different scents may even have been used along the maze to tell the current time with a puff. And there were even incense alarm clocks where incense burned through strings and dropped metal balls into a bowl to sound the alarm.
Incense timers were used in China in the 21st century by coal miners to gauge 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 6th century, when the poet Yu Jianwu wrote:
By burning incense [we] know the hour of the night,
With graduated candle [we] confirm the countdown of the watch.
The Incense Clock takes the core concept – combustion timing – and elevates it to a magnificent new level of complexity. When examining the example held by the Science Museum, I was struck by its small size: no bigger than a cup of coffee. Yet its small compartments are neatly packed with everything it needs to function. In the bottom tray, you’ll find a bite-sized shovel and shock absorber; above, a pot of wood ashes to trace the path of the incense; then, stacked on top, a set of stencils for tracing the mazes. As Silvio Bedini, historian of scientific instruments, explains in his in-depth study of the use of fire and incense for the measurement of time in China and Japan, the variety allows for seasonal variations: longer paths to burn through the endless winter nights, while shorter paths serve for the summer.
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