82 The Realm of Nature CHAP. 



the Sun always appears higher in the sky than its true 

 position ; the period of daylight is thus increased and the 

 period of darkness diminished, the effect being greatest in 

 high latitudes. 



LENGTH OF THE LONGEST DAY. 



Latitude o TO 20 30 40 50 60 70 80 90 



Hours 12 i2h. 35 130. 12 1311.56 1411.51 i6h. 19 i8h. 30 65 days i6id. i86d. 



(Refraction slight and (Refraction 



not allowed for) allowed for) 



Between the tropics the Sun is vertical in every latitude 

 twice in the year ; outside the tropics never. Even in 



summer the altitude of the 

 Sun is low in high latitudes ; 

 it can never be more than 

 23^ at the poles, nor more 



ABODE 



The amount of radiant 



F ' G bread,I of"fh e e Cea^E $. J eneigy falling on the surface 



as that of AB, but striking at an angle varies with the altitude of 

 of 30 the length CE is twice the length ,1 c 17 ' T y V, 



AB where the rays fall perpendicularly. tne ^Un. Fig, I 8 Shows 



that the same beam of light 



which, falling vertically, covers I sq. ft. of surface, will, 

 when falling at an angle of 30 cover 2 sq. ft., and so 

 produce on each square foot only one half of the effect of 

 vertical light ; at a lower angle the heating effect of sun- 

 light is very slight. Oblique rays of light also pass through 

 a thicker layer of the Earth's atmosphere, and so are more 

 absorbed than vertical rays. 



125. Zones of Climate. It follows that the region 

 between the tropics receives most of the solar energy, 

 higher latitudes sharing it in smaller and smaller pro- 

 portions. The Earth has consequently been divided into 

 zones of climate a word originally meaning inclination of 

 the Sun's rays. The areas within the polar circles, poorest 

 in radiant energy, are termed the Frigid Zones, those 

 between the polar circles and the tropics, where there is a 

 tolerable abundance of radiation, the Temperate Zones, and 

 the wide belt between the tropics which is overflowing with 



