περιοικοι, 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 noonday with the one it is midnight with the other, there being twelve hours in an east and west direction. These are found on the globe by the hour-index, or by turning the globe half round, that is, 180 degrees either way.