INTRODUCTION TO ASTRONOMY. 

 Fig. 12. 



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sun's rays quite freely ; and besides, it is always loaded more or less with 

 dense and foggy vapour, which the rays of the sun cannot easily penetrate ; 

 therefore the greater the quantity of atmosphere the sun's rays have to 

 pass through in their way to the earth, the fewer of them will reach it. 

 This will be better understood by referring to Jig. 14. The dotted line 

 round the earth describes the extent of the atmosphere, and the lines which 

 proceed from the sun to the earth, the passage of two equal portions of 

 the sun's rays to the equatorial and polar regions : the latter, from its 

 greater obliquity, passes through a greater extent of atmosphere. 



The diminution of heat, morning and evening, is also owing to the 

 greater obliquity of the sun's rays ; and, as such, they are affected by 

 both the causes which have just been explained : the difficulty of passing 

 through a foggy atmosphere is more particularly applicable to them, as 

 mists and vapours are very prevalent about the time of sunrise and sunset. 

 But the diminished obliquity of the sun's rays is not the sole cause of the 

 heat of summer ; the length of the days greatly conduces to it ; for the 

 longer the sun is above the horizon, the more heat he will communicate 



Fig. 14. 



