This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Candles can be easily fitted with attachments to extinguish the light at a set time. To determine the length of time, it is necessary to mark a candle of the size used and time how long a certain length of it will burn. Then it is sufficient to suspend a small metal dome, or cap, to which a string is attached, d i -rectly over the flame, and run the opposite end of the string over nails or through screw eyes, so that it can be tied around the candle such a distance from the flame end, that the part between the flame and the string will be consumed in the time desired for the light to burn. When this point is reached, the string slips off the candle, and the cap drops on the flame.