Candle clocks are one of the ways that ancient people attempted to use natural phenomenon to keep accurate record of the time during the day. They consist of a long thin candle, which is marked at even intervals. When the candle is lit it burns down at a relatively steady pace, and by keeping track of which mark the flame is at, you can get a rough estimate of how much time has passed since it was lit.
It is almost impossible to know when the first candle clock was used, as the practice of marking candles may have been in use throughout the world for centuries before it was ever recorded. Our first written reference to the use of a kind of candle clock comes from a poem written by Chinese thinker You Jiangu, around the year 520 AD. In the work, the candle clock is referenced as a way of telling what time it was during the night, when the sun could not be used. We also have evidence of these candles being used in Japan until about the 10th century AD.
The most famous candle clock was used by King Alfred the Great of England. This clock consisted of six candles, each crafted to be exactly twelve inches in height, and precisely uniform in width. These candles each had 12 marks, with each mark representing twenty minutes, and the entire candle set to burn away every four hours. At four hours each the 6 candles were able to maintain a steady reading of the time, for a full 24 hour period, without being replenished. The candles themselves were protected by wooden frames with glass paneling that allowed the time to be monitored.
In 1206 Al-Jazari fashioned what are considered the most complex and ingenuous candle clocks ever made. These clocks used weight and counterbalance, to move a series of automata which displayed a read out of the time. As the candle burned, it became lighter, and the counterweights would move, allowing the device to operate. These clocks were the first to use a bayonet fitting, which is a fastening mechanism which is still employed to this day.
While candle clocks were relatively accurate, it was still impossible to completely control the speed at which the flame would burn and the max would melt. The fact that the max would melt unevenly could also cause problems with the accuracy of these timepieces. They were generally abandoned in favor of mechanical clocks once they became widely available.