Fire clock
Fire burns almost as steadily as water flows, and it doesn't need refilling. Make the fuel the same along its whole length and it becomes the scale. In China they turned this into an alarm clock.
- 6 candlesa day burned by Alfred the Great, four hours each
- 12 inchesthe length of each marked candle
- 1 stickthe unit of a Japanese geisha's fee
Time that burns away
In the Buddhist monasteries of China incense was burned during meditation and prayer, and people soon noticed that a stick of the same kind always burned for the same time. From there it was one step to a clock: mark the stick.
On the right is a "dragon boat" incense alarm clock. Time is sped up in the drawing: one second for six minutes.
Threads and balls
The stick lay in the groove of a wooden boat, and threads with metal balls on their ends were tied across it at equal intervals. When the fire reached a thread it burned through, and the balls fell into a brass tray with a chime.
The owner chose which mark to hang the balls on, and so was woken for morning prayers or the start of work.
Thin sticks burn faster
The burning rate depends on the stick's thickness and recipe, so clock sticks were made strictly to one formula: scented powder and binder, always in the same proportions and the same thickness.
We have taken a thin stick: it smoulders noticeably faster. Move the pointer or swipe sideways over the drawing to change the thickness.
The enemy of fire clocks
Wind fans the glowing tip, and the stick burns faster. In our model a draught speeds the burning by 60%: the chime comes early.
Europe knew the problem too. According to his biographer Asser, King Alfred the Great of England burned six wax candles a day in the 9th century, each twelve inches long and marked. To stop the draughts in churches and tents from driving the flame, the candles were set in lanterns of finely scraped horn.
A maze of powder
Some fire clocks had no sticks at all. A trail of scented powder was laid through a stencil on a smooth bed of ash, in the shape of a character or a maze. The fire crept along it for hours, and the turns it had passed told the time. Such patterns were called incense seals.
We have lit the stick again; the "Light again" button on the drawing does the same.
The last fire clocks
In Japan fire clocks lasted especially long. In the teahouses a geisha's fee was counted in burnt sticks of incense: light a new one, and a new stretch of time began.
Fire clocks lost to mechanical ones not in accuracy but in convenience: they had to be lit and reloaded with fuel every time. Still, the alarm with its balls is one of the earliest ways in history to wake at a chosen hour without a watchman or a bell.