356 PHYSIOGRAPHY. [chap. 



At the equator, the sun is directly overhead, or in the 

 zenith, at noon in the spring and autumn, and is never more 

 than 23^" from the zenith at either solstice ; while the days 

 and nights are of practically equal length all the year round. 

 Within a circle of 23^° latitude, on each side of the equator, 

 there is a belt called the torrid or tropical zone. At everj' 

 place within these zones, the sun is in the zenith twice 

 a year, and is never more than 47° from the zenith- 

 Hence the intense heat of tropical regions. The boundaries 

 of these zones are called tropics ; and the countries just out- 

 side these circles form sub-tropical rtgions. 



Around each pole, a circle of 23^° radius may be drawn, 

 which will include the polar regions or frigid zone. The 

 Northern circle is called the Arctic, and the southern is 

 called the Antarctic circle. At the poles themselves, the sun 

 is above the horizon for six months continuously, and below 

 it for an equal period. But, though the polar day is so long, 

 the extreme obliquity of the rays prevents the continued 

 sunshine from having so great a heating power as it would 

 have further south. In fact, at the poles, the sun never 

 rises more than 23^° above the horizon. 



Between the polar and the tropical zones, there is, in each 

 hemisphere, a broad belt of the earth's surface, which is 

 known as the temperate zone. The distribution of the 

 surface of the globe into zones is shown in Fig. 116. 



These zones are distinguished, as has just been explained, 

 by their differences of climate. The principal factor in the 

 formation of climate is, of course, solar heat ; the climate of 

 any place depending, primarily, on the lengths of the days 

 and nights, and on the relative duration of the seasons. But 

 climate is also greatly affected by the nature of the surface, 

 whether it be land or water. Water parts with its heat 

 much more slowly than the land does, and it thus retains a 



