(see Curfew, Encycl.), called in the law Latin of the middle ages ignitigium or pyritegium, and in French, curvetu—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 of earth. This practice still prevails among the cottagers in some parts of Scotland, and we doubt not of other countries. In the dark ages, when all ranks of people were turbulent, a law was almost everywhere 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 ignitigium. The law of William the Conqueror, which introduced this practice into England, as has been mentioned in the Encyclopaedia, was abolished by Henry I. 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 XXIII., with a view to avert certain apprehended misfortunes, which rendered his life uncomfortable, gave orders, that every person, on hearing the ignitigium, should repeat the Ave Maria three times. When the appearance of a comet, and a dread of the Turks, afterwards alarmed all Christendom, Pope Calixtus III. increased these periodical times of prayer, by ordering the prayer-bell to be rung also at noon. Beckmann's History of Inventions.

CURVE or EQUABLE APPROACH. It was first proposed by Leibnitz, namely, to find a curve, down which a body descending by the force of gravity shall make equal approaches to the horizon in equal portions of time. It has been found by Bernoulli and others, that the curve is the second cubical parabola, placed with its vertex uppermost, and which the descending body must enter with a certain determinate velocity. Varignon rendered the question general for any law of gravity, gravity, by which a body may approach towards a given point by equal spaces in equal times. And Maupertius also resolved the problem in the case of a body descending in a medium which resists as the square of the velocity.