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POLYBIUS supposes six zones: two situated between the poles and the arctic circles; two between the arctic circles and the tropics; and two between the tropics which are divided by the equator. However, it appears to me that the division into five zones accords best both with the order of external nature and geography. With external nature, as respects the celestial phenomena, and the temperature of the atmosphere. With respect to the celestial phenomena, as the Periscii and Amphiscii are thereby divided in the best pos- sible manner, and it also forms an excellent line of separation in regard to those who behold the stars from an opposite point of view.1 With respect to the temperature of the atmosphere, inasmuch as looked at in connexion with the sun, there are three main divisions, which influence in a remarkable degree both plants, animals, and every other animated thing, existing either in the air, or exposed to it, namely, excess of heat, want of heat, and a moderate supply of heat. In the division into [five] zones, each of these is correctly distinguished. The two frigid zones indicate the want of heat, being alike in the temperature of their atmosphere; the temperate zones possess a moderate heat, and the remaining, or torrid zone, is remarkable for its excess of heat. The propriety of this division in regard to geography is equally apparent; the object of this science being to determine the limits of that one of the temperate zones which we in- habit. To the east and west, it is true, the boundaries are formed by the sea, but to the north and south they are indicated by the atmosphere; which in the middle is of a grate- ful temperature both to animals and plants, but on either side is rendered intemperate either through excess or defect of heat. To manifest this threefold difference, the division of tile globe into five zones becomes necessary. In fact, the division of the globe, by means of the equator, into two hemispheres, the one northern, wherein we dwell, and the other southern, points to this threefold division, for the regions next the equator and torrid zone are uninhabitable on account of the heat, those next the poles on account of the cold, but those in the middle are mild, and fitted for the habitation of man. Posidonius, in placing two zones under the tropics, pays no regard to the reasons which influenced the division into five zones, nor is his division equally appropriate. It is no more than if he were to form his division into zones merely according to the [countries inhabited] by different nations, calling one the Ethiopian, another the Scythian and Keltic,2 and a third the Intermediate zone.
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