Candle clocks have existed for many centuries, the first recorded date being in the sixth century and Alfred the Great had a six candle version.
They generally consist of a tall slim candle with graduated markings often with numbers. In some versions the candle could be held in a case in which the markings were engraved. When the candle is lit it burns at a known steady rate so by noting the mark at which the flame has reached an estimate can be made of the time which has elapsed since the candle was lit.
A simple refinement involves inserting a nail or the object at a particular mark so that when reached by the flame it fell out into a plate alerting the user of the time. This type of candle clock which is no longer in existence was mainly of use during the night or on very overcast days.