A candle clock can be used to measure time. In the past it was used to measure time at night or on cloudy days. There are marks on the side of the candle. As the candle burns down and the marks disappear you can measure the time that has passed.
The marks on a candle are set to measure the passing of the hours. When a candle has burnt the wax between two marks an hour has passed. The candle needs to burn steadily. If it is in a draught it will burn more quickly so it is placed in a holder which lets you see the marks on the candle yet protects the candle from draughts.
The spacing of the marks on the candle is worked out in the following way. Two candles are set up next to another device which measures time. Today it would be a mechanical clock but in the past it could have been a sundial or a sand timer. One of the candles is lit and when an hour has passed the height of the candle is compared with the unlit candle and a mark is made in the side of the unlit candle. After another hour has passed a second comparison is made and a second mark is made in the unlit candle. This procedure is repeated every hour until the first candle burns out. When this happens the second candle has a series of marks along its side which measure the passing of the hours. This is now used like a ruler to put markings on other similar candles to turn them into candle clocks.
The night light is a kind of modern candle clock. This is a short fat candle which is designed to burn for eight hours.