test and coldest seasons being separated by a period of half a year* 

 differ very considerably from each other in their temperature. 

 The whole terrestrial zone lying between these two tropics is 

 called the hot zone, or torrid zone. When the sun's distance from 

 the equator north is the greatest possible, i. e., when it is in the 

 tropic of Cancer or at the point VI, see figure, page 80, the north 

 pole of the earth is illuminated, and the south pole in darkness, as- 

 represented in the figure, page 109. If we suppose a circle traced 

 upon the earth as shown at c d, it is evident that as the sun 

 now illuminates all within this circle, the day will be to a specta- 

 tor situated upon it, 24 hours in length, or in other words visible 

 during a complete revolution of the earth on its axis. A similar 

 circle shown at g /i, indicates the position in the southern 

 hemisphere where the longest day is 24 hours. These two cir- 

 cles are called, the former the Arctic, and the latter the Antarctic,, 

 Hie former is situated 23 23' from the north pole, and conse- 

 quently 6G 32' north of the equator, and the latter at the same 

 distance from the south pole, and south of the equator. The 

 terrestrial zones included between the tropics and the polar cir- 

 cles, are called the northern and southern temperate zones. The 

 four seasons of the year are most strongly characterized in these 

 zooies, and the general rule for the di munition of heat is, directly 

 n.s tke distance from the equator. Within the polar circles are 

 the northern and southern frigid zones. As the earth turns upon 

 its axis from west to east, the sun is apparently caused to rise in 

 the east, move over the heavens, and set in the west, thus pro- 

 ducing the alternation between day and night. During the day, 

 the surface of the earth is warmed by the rays of the sun, but 

 when these are withdrawn at night, the heat is radiated to the 

 heavens and lost, daring the night therefore the surface of the 

 earth is cooled. We shall presently see that the vicissitudes in 

 ciiraate varying with the latitude, are mainly due to the unequal 

 longths of day and nigh?. Under the equator the days and nights 

 are very nearly equal, throughout the year, each lasting 12 hours. 

 As soon, however, as we leave the equator, the length of the day 

 varies according to the season of the year, and the difference be- 

 tween the day and night, becomes more striking as we approach 



