in Chronology, an aliquot part of a natural day, usually a 24th, but sometimes a 12th. The origin of the word hora, or æra, comes, according to some authors, from a furnace of the sun, the father of hours, whom the Egyptians call Horus. Others derive it from the Greek ἀπέλας, to terminate, distinguish, &c. Others from the word υγρος, urine; holding, that Trismegistus was the first that settled the division of hours which he did from observation of an animal consecrated to Serapis, named cynocephalus, which makes water 12 times a-day, and as often in the night, at equal intervals.

An hour, with us, is a measure or quantity of time, equal to a 24th part of the natural day, or nythemon; or the duration of the 24th part of the earth's diurnal rotation. Fifteen degrees of the equator answer to an hour; though not precisely, but near enough for common use. It is divided into 60 minutes; the minute into 60 seconds, &c.

The division of the day into hours is very ancient; as is shown by Kircher, Oedip. Egypt. tom. ii. p. ii. cl. viii. c. 8.: though the passages he quotes from Scripture do not prove it.—The most ancient hour is that of the 12th part of the day. Herodotus, lib. ii. observes, that the Greeks learnt from the Egyptians, among other things, the method of dividing the day into twelve parts.—The astronomers of Cathaya, &c. Bishop Beveridge observes, still retain this division. They call the hour chag; and to each chag give a peculiar name, taken from some animal: The first is called zeth, "mouse;" the second, chiu, "bullock;" the third, zem, "leopard;" the fourth, mau, "hare;" the fifth, chiu, "crocodile," &c.

The division of the day into 24 hours, was not known to the Romans before the first Punic war.—Till that time they only regulated their days by the rising and setting of the sun. They divided the 12 hours of their day into four, viz. prime, which commenced at six o'clock; third, at nine; sixth, at twelve, and none, at three. They also divided the night into four watches, each containing three hours.