This section is from the book "A Library Of Wonders And Curiosities Found In Nature And Art, Science And Literature", by I. Platt. Also available from Amazon: A library of wonders and curiosities.
The curfew bell (called, in the low Latin of the middle ages, ignitegium, or peritegiun, and in French, couvrefew) was a signal for all persons to extinguish their fires at a certain hour. In those ages, people made fires in their houses in a hole or pit in the centre of the floor, under an opening formed in the roof; and when the fire was burnt out. or the family went to bed, the hole was shut by a cover of wood or earth. This practice still prevails among the cottagers in some parts of Scotland, and perhaps in other parts of the kingdom. In the dark ages, when all ranks of people were turbulent, a law was almost every where established, that the fire should be extinguished at a certain time in the evening; that the cover should be put over the fire-place, and that all the family should retire to rest, or at least keep within doors. The time when this ought to be done, was signified by the ringing of a bell, called therefore the curfew bell, or tgnitegium. This was the law of William the Conqueror, who first introduced the practice into England, and which was abolished by Henry the First, in 1100.
The ringing of the curfew bell gave rise to the Prayer Bell, as it is called, which is still retained in some Protestant countries. Pope John the Twenty-third, with a view to avert certain apprehended misfortunes which rendered his life uncomfortable, gave orders, that every person, on hearing the ignitegium, should repeat the Ave Maria three times.
When the appearance of a comet, and the dread of Turks, alarmed all Christendom, Pope Calixtus the Third increased these periodical times of prayer, by ordering the prayer bell to be rung also at noon.