THE SURFACE OF THE EARTH. 



sun, from March to June; the days during that interval are 

 gradually longer and warmer. It is constantly less inclined 

 from June to September; the days during that interval are 

 gradually shorter and less warm. 



Hence it is, that in* the northern hemisphere the longest days 

 and highest temperature take place after June, the temperature 

 after March being more moderate. The interval between March 

 and June constitutes, therefore, the spring, and the interval 

 between June and September, when the accumulated effects 

 of heat are greater, the summer. 



151. This varying position of the earth towards the sun will be 

 rendered more easily intelligible by illustrative diagrams. 



Let x s in these four figures represent the axis of the earth, 

 x being the north, and s the south pole. Let E Q, at right angles 

 to N s be the equator, and let s' be the direction of the sun. 



152. In fig. 8 is shown the position of the earth on the 21st of 

 March, the day of the spring equinox. The equator E is then 

 presented exactly in the direction of the sun, the light and heat 

 of which are equally distributed between the two hemispheres. 



Fig. 9. 



Fig. 10. 



S 



21st September. 21st December. 



The boundary of the enlightened hemisphere passes through the 

 poles N and s, and divides into two equal parts all the parallels 

 of latitude. As the earth revolves, therefore, upon its axis, each 

 place upon its surface is during equal intervals exposed to and 

 withdrawn from the sun's light. In other words, the days and 

 nights are equal in all parts of the earth. 



