THOUGHTS ON NATURAL PHILOSOPHY. 17 



the nebular theory (though here and there there are 

 exceptional phenomena) astronomers have generally 

 regarded this theory with considerable approval. 

 *, There are very remarkable features in the solar 

 system which point unmistakably to some common 

 origin of many of the different bodies which it 

 contains. We must at once put the comets out of 

 view. It does not appear that they bear any testi- 

 mony on either side of the question. We do not 

 know whether the comets are really indigenous to 

 the solar system or whether they may not be merely 

 imported into the system from the depths of space. 

 Even if the comets be indigenous to the system, 

 they may, as many suppose, be merely ejections 

 from the sun, or in any case their orbits are exposed 

 to such tremendous perturbations from the planets 

 that it is quite unsafe from the present orbit of a 

 comet to attempt any estimate of what that orbit 

 may have been countless ages ago. On all these 

 grounds we must put the comets on one side for the 

 present, and discuss the nebular theory without any 

 reference thereto. But even with this omission we 

 still muster in the solar system from two to three 

 hundred bodies almost every one of which pronounces 

 distinctly, though with varying emphasis, in favour 

 of the nebular theory. The first great fact to which 

 we refer is the common direction in which the 

 planets revolve around the sun. This is true not 

 only of the great planets Mercury, Venus, the Earth, 

 Mars, Jupiter, Saturn, Uranus, and Neptune; it is 

 also true of the host of more than two hundred small 

 planets. All these bodies perform their revolution 



