78 REVOLUTION OF THE EARTH. 



off. The northern hemisphere is now presented to the 

 s\m, and as here we have the largest portion of dry land, 

 the powers of absorption are at their maximum. 



The autumnal equinox has arrived, with its equal day 

 and night, as in the spring, but now the whole northern 

 hemisphere is opposite the. sun ; hence, according to the 

 laws already explained, we see the causes of the increased 

 heat of the autumnal season. 



The winter solstice has come, with its long night and 

 shortened day. The time during which radiation is 

 going on is nearly twice that in which absorption takes 

 place, and the earth is in her worst position for receiv- 

 ing heat, as that hah which has the largest surface of 

 water is towards the sun. 



These are the causes which lead to the variations of 

 the seasons, and through these we learn why we are 

 colder when near the sun than when at a considerably 

 greater distance. 



An analysis of the spectrum shows us that there are 

 some changes regularly taking place in the state of the 

 solar beam, which cannot be referred to the mere alte- 

 ration of position. It may be inferred, from facts 

 by long-continued observations, that the three classes 

 of phenomena light, heat, and chemical power, distin- 

 guished by the term Actinism which we detect in the 

 sun's rays, are constantly changing their relative propor- 

 tions. In spring, the chemical agency prevails; in 

 summer, the luminous principle is the most powerful ; 

 and in the autumn, the calorific forces are in a state of 

 the greatest activity.* The importance of these varia- 

 tions, to the great economy of vegetable life, will be 

 shown when we come to examine the phenomena con- 

 nected with organisation. 



* A particular examination of this curious question will bo 

 found in the Author's report On the Influence of the Solar Rays on 

 the Growth of Plants : Eeports of the British Association for 1847.. 



