PRESSURE OF LIGHT IN ASTRONOMY 8 1 



larger bodies for which gravitative pull exceeds 

 light-pressure we have three kinds of effect 



1. A lengthening of their period of revolution. 



2. A tendency to make the orbit more circular. 



3. A drawing in, in a spiral, ending sooner or 

 later in the sun. 



On bodies as large as the planets these effects 

 are inappreciable in any time which we need 

 consider ; but on bodies of radius from I cm. 

 downwards, the effects might be measurable. One 

 result at once appears. Our system is now full 

 of such bodies, if we are right in assuming that 

 many of the shooting stars are small particles. 

 Whatever age we ascribe to the Sun it must be 

 vastly longer than the time required to have drawn 

 all such particles originally contained in his system 

 into himself. The supply must therefore be con- 

 tinually renewed, and we are led to at least 

 a probable conclusion that it is renewed from the 

 space outside our system. There is some ground 

 for supposing that we draw some, at least, of 

 the comets from outside, and if we suppose that 

 they consist of clouds of such particles, we can see 

 that the actions which we have here considered 

 would gradually lead to their disintegration, even 

 if no other action tended in the same direction. 

 There would be in the first place a tendency of 

 the smaller particles to lag behind in the orbit. 

 Then a tendency for all the particles to move in 

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