Periodical, something that terminates and comprehends a period; such is a periodic month; being the space of time wherein the moon dispatches her period.

Perioeci, περιοίκοι, in geography, such inhabitants of the earth as have the same latitudes, but opposite longitudes, or live under the same parallel and the same meridian, but in different semicircles of that meridian, or in opposite points of the parallel. These have the same common seasons throughout the year, and the same phenomena of the heavenly bodies; but when it is noon-day with the one, it is midnight with the other, there being twelve hours in an east and west direction. They are found on the globe by the hour-index, or by turning the globe half round, that is, 180 degrees either way.