NEWTON S PRINCIPJA. 323 



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Laplace deduces from this that the zodiacal light cannot 

 be part of the Sun s atmosphere, for it has the form of a 

 very flat lens, in which the polar diameter is far less than 

 two-thirds the equatorial. Another sufficient reason is, 

 that an atmosphere cannot extend beyond the orbit of a 

 planet which describes its revolution in a time equal to 

 the rotation of the Sun. Hence, as the Sun revolves in 

 twenty-five days and a half, its atmosphere cannot extend 

 so far as Mercury or Venus. We know that the zodiacal 

 light extends much further. It is therefore not part of 

 the Sun s atmosphere. The zodiacal light is a lenticularly 

 shaped envelope which revolves round the Sun. Her- 

 schel ( 897.) conjectures it to be no more than the denser 

 part of that medium which resists the motion of comets, 

 loaded perhaps with the actual materials of the tails of 

 millions of those bodies, of which they have been stripped 

 in their successive perihelion passages. It is an illuminated 

 shower or tornado of stones. According to Professor 

 Thomson, the inner parts of this tornado are always getting 

 caught by the resistance of the Sun s atmosphere, and 

 drawn to his mass by gravitation. They are always 

 approaching the Sun, but very gradually, and he asserts 

 that the mere fall of these aerolithes on the Sun is suffi 

 cient to account for the permanence of the Sun s heat. 



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