HOUR

, in Chronology, an aliquot part of a natural day, usually the 24th, but sometimes a 12th part. With us, it is the 24th part of the earth's diurnal rotation, or the time from noon to noon, and therefore it answers to 15 degrees of the whole circle of longitude, or of 360°. The hour is divided by 60ths, viz, first into 60 minutes, then each minute into 60 seconds, &c.

The division of the day into Hours is very ancient; as is shewn by Kircher, Oedip. Ægypt. tom. 2, par. 2, class 7, cap. 8. The most ancient Hour is that of the 12th part of the day. Herodotus, lib. 2, observes, that the Greeks learnt from the Egyptians, among other things, the method of dividing the day into 12 parts. And the astronomers of Cathaya, &c, still retain this division.|

The division of the day into 24 hours, was not known to the Romans before the 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 6 o'clock, Third at 9, Sixth at 12, and None at 3. They also divided the night into four watches, each containing 3 Hours.

There are various kinds of Hours, used by chronologers, astronomers, dialists, &c. Sometimes too,

Hours are divided into Equal and Unequal.

Equal Hours, are the 24th parts of a day and night precisely; that is, the time in which the 15 degrees of the equator pass the meridian. These are also called Equinoctial Hours, because measured on the equinoctial; and Astronomical, because used by astronomers.

Astronomical Hours, are equal Hours, reckoned from noon to noon, in a continued series of 24.

Babylonish Hours, are equal Hours, reckoned from sun-rise in a continued series of 24.

European Hours, used in civil computation, are equal Hours, reckoned from midnight; 12 from thence till noon, and 12 more from noon till midnight.

Jewish, or Planetary, or Ancient Hours, are 12th parts of the artificial day and night. They are called Ancient or Jewish Hours, because used by the ancients, and still among the Jews. They are called Planetary Hours, because the astrologers pretend, that a new planet comes to predominate every Hour; and that the day takes its denomination from that which predominates the first Hour of it; as Monday from the moon, &c.

Italian Hours, are equal Hours, reckoned from sunset, in a continued series of 24.

Unequal or Temporary Hours, are 12th parts of the artificial day and night. The obliquity of the sphere renders these more or less unequal at different times; so that they only agree with the equal Hours at the times of the equinoxes.

Hour-Circles, or Horary-Circles, are great Circles, meeting in the poles of the globe or world, and crossing the equinoctial or equator at right angles; the same as meridians. They are supposed to be drawn through every 15th degree of the equinoctial and equator, each answering to an hour, and dividing them into 24 equal parts; and on both globes they are supplied by the meridian Hour-circle and index.

Hour-Glass, a popular kind of chronometer or clepsydra, serving to measure time by the descent or running of sand, water, &c, out of one glass vessel into another. —The best, it is said, are such as, instead of sand, have egg-shells, well dried in the oven, then beaten fine and sifted.

Hour-Lines, on a Dial, are lines which arise from the intersections of the plane of the Dial, with the several planes of the Hour-circles of the sphere; and therefore must be all right lines on a plane Dial.

Hour-Scale, a divided line on the edge of Collins's quadrant, being only two lines of tangents of 45 degrees each, set together in the middle. Its use, together with the lines of latitude, is to draw the Hour-lines of Dials that have centres, by means of an equilateral triangle, drawn on the dial-planes.