Ancient clocks and old Technique of alarms

In old times when there were no clocks there were candles used as clocks, the time was guessed by the amount of melted candle. If any one wanted a reminder or alarm, he pushed a nail at any point and glass dish or plate was placed  and when the flame reaches to that point the candle melts and the nail falls to the glass dish producing sound of fall and hence it is used as alarm or reminder.