432 ASTRONOMY. 



no violent elementary heat existing in the rays of 

 the sun themselves essentially, but that they pro- 

 duce heat only when they come into contact with 

 the planetary bodies. 



Several years after this, Herschell published his 

 theory of the nature of the sun, which is briefly as 

 follows. He considers the sun as a most magnifi- 

 cent habitable globe, surrounded by a double set 

 of clouds ; those which are nearest its opaque 

 body are less bright, and more closely connected 

 together, than those of the upper stratum, which 

 form the luminous apparent globe we behold : this 

 luminous external matter is of a phosphoric nature, 

 having several accidental openings in it, through 

 which we see the sun's body, or the more opaque 

 clouds beneath : these openings form the spots we 

 see. 



Mercury. — This planet being the nearest to the 

 sun, and the least in magnitude, is very seldom 

 visible. It never appears more than a few degrees 

 from the sun's disc, and is generally lost in the 

 splendour of the solar beams. On this account, 

 astronomers have had few opportunities of making 

 accurate observations upon it ; no spots have been 

 observed upon it, consequently, the time of its 

 rotation on its axis is not known. Being an infe- 

 rior planet, it consequently must show phases like 

 the moon ; and it never appears quite full to us. 

 It is seen sometimes passing over the sun's disc, 

 which is called its transit. 



Venus is the brightest and largest to appearance 

 of all the planets, and is distinguished from the 

 rest by her superiority of lustre. It is generally 

 called the morning or evening star, according as it 

 precedes or follows the apparent course of the sun. 

 Some have thought that they could discover spots 



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