Candle clocks

(first recorded mention late 9th century AD; probably much older)

Among the earliest human inventions, candles provided another way to tell time indoors, at night, or on a cloudy day. Like water clocks, candle clocks couldn't be used to find the time, but the sides of candles could be marked to indicate the passage of predetermined periods of time.

King Alfred the Great of England has been credited with inventing graduated candles in the late 9th century to divide his day into equal periods of study and prayer, royal duties, and rest. Before candle clocks made an appearance in Europe, however, it is likely that they were in use in the East, as were sundials and water clocks.

During the Sung dynasty in China (960-1279), calibrated candles and sticks of incense measured time. In one 18th- or 19th-century incense clock, six threads with weights on either end were draped over an incense stick at regular intervals. As the incense burned, the threads burned one by one and the weights dropped to a sounding plate below. Sticks of incense with different scents might be used at different times, so that the hours were marked by a change in fragrance.

A candle clock could be transformed into a timer by sticking a heavy nail into the candle at the mark indicating the desired interval. When the wax surrounding the nail melted, the nail clattered onto a plate below.